When I was a bachor in yeshiva we advised not to go the chupah of Conservative or Reform weddings to avoid providing valid witnesses
Rav Sternbuch(2:625): Question: A Baal Teshuva who parents married without chupah or Kiddushin – it is desirable for him to encourage them to get married according to the halacha? Answer: It is clear that living together without chupah and Kiddushin is a degradation and blemish to the family and to encourage them get halachically married would save them from halachic prohibitions. However contemporary irreligious couples have totally rejected any commitment to mitzvos and they swap wives and commit other types of severe sins. If his parents are of this type then there is absolutely no mitzva to have them married according to the halacha. This is especially true according to their son that they don’t have a good marriage. Thus it is questionable whether marriage is a good idea since there is a real possibility that they will separate without writing a Get. Therefore if they were halachically married this would result in their transgressing the prohibition of adultery. Thus the benefits of marriage would actually cause a loss because it would cause them to sin. Therefore whether an irreligious couple should marry according to the halacha is dependent on the quality of their relationship. If it is clear that they have a solid relationship – even if they don’t keep the laws of nidah – then it might be permissible for their son to encourage them to get married according to the halacha. Because they will be living together anyway so at least this will save them from the blemish to their family of not being halachically married. However in the case before us there is a strong concern that if they get married according to the halacha it will lead to the more severe problem of adultery. In such a case there is absolutely no mitzva for them to marry properly. In fact the opposite is true because it will lead them to transgress severe prohibitions. Therefore with the irreligious there are many times that it is better for them to live together without a Jewish marriage – because if they would have chupah and Kiddushin it would lead to producing mamzerim or sofek mamzerim. The halachic rule that most intercourse is with the husband would not apply to those who are licentious as we see concerning a Sotah. This is the view of the Shulchan Aruch (E.H. 4:15). Therefore before encouraging them to have a halachic wedding – their relationship needs to be investigated thoroughly. Each case needs to determine whether it is to their benefit to be married and thus that which leads to minimize sin should be done. Consequently it is a good idea to consult with the local rabbi who knows their circumstances well. You should know that in prior generations the non-observant would degenerate by means of reading secular books that involved nonsense or heresy etc. This would lead to rejecting the observance of mitzvos and they would descend to various degrees of impurity – according to their sins.
Rav Sternbuch(2:625): Question: A Baal Teshuva who parents married without chupah or Kiddushin – it is desirable for him to encourage them to get married according to the halacha? Answer: It is clear that living together without chupah and Kiddushin is a degradation and blemish to the family and to encourage them get halachically married would save them from halachic prohibitions. However contemporary irreligious couples have totally rejected any commitment to mitzvos and they swap wives and commit other types of severe sins. If his parents are of this type then there is absolutely no mitzva to have them married according to the halacha. This is especially true according to their son that they don’t have a good marriage. Thus it is questionable whether marriage is a good idea since there is a real possibility that they will separate without writing a Get. Therefore if they were halachically married this would result in their transgressing the prohibition of adultery. Thus the benefits of marriage would actually cause a loss because it would cause them to sin. Therefore whether an irreligious couple should marry according to the halacha is dependent on the quality of their relationship. If it is clear that they have a solid relationship – even if they don’t keep the laws of nidah – then it might be permissible for their son to encourage them to get married according to the halacha. Because they will be living together anyway so at least this will save them from the blemish to their family of not being halachically married. However in the case before us there is a strong concern that if they get married according to the halacha it will lead to the more severe problem of adultery. In such a case there is absolutely no mitzva for them to marry properly. In fact the opposite is true because it will lead them to transgress severe prohibitions. Therefore with the irreligious there are many times that it is better for them to live together without a Jewish marriage – because if they would have chupah and Kiddushin it would lead to producing mamzerim or sofek mamzerim. The halachic rule that most intercourse is with the husband would not apply to those who are licentious as we see concerning a Sotah. This is the view of the Shulchan Aruch (E.H. 4:15). Therefore before encouraging them to have a halachic wedding – their relationship needs to be investigated thoroughly. Each case needs to determine whether it is to their benefit to be married and thus that which leads to minimize sin should be done. Consequently it is a good idea to consult with the local rabbi who knows their circumstances well. You should know that in prior generations the non-observant would degenerate by means of reading secular books that involved nonsense or heresy etc. This would lead to rejecting the observance of mitzvos and they would descend to various degrees of impurity – according to their sins.
However when it came to getting married they were careful to do it within the framework of halacha and therefore the majority came to the beis din that was concerned with marriage and divorce. However today the secular are that way - not because of intellectual concerns - but simply from lust. Their basic desire is to have a totally unrestricted life for the sole purse of pleasure. Thus they live like wild animals with mocking authority, lacking any moral restraint or limits with focus entirely on pleasure and parties. And this attitude applies also to their wives where moral conduct or commitment is lacking – so each one simple does what gives them pleasure. Therefore one should be careful to avoid Kiddushin for those who are likely to be involved in adultery. On the other hand in our day when these hedonistic people hit bottom and they see that their life is totally worthless they are likely to think about repenting and find a purpose in their life. This is in contrast with previous generations who were ideological heretics that they would be loyal to their heresy and would not repent even at the gates of Hell. The bottom line is that it is impossible for us to make a general rule but each case needs to be examined separately. G‑d should return them and all Jews with complete repentance. In practice a number of gedolim in Israel have expressed concerns that they might totally reject halacha after the wedding such as on some kibbutzim where they swap wives. They have suggested that perhaps it is better to mislead them into thinking they have had a halachic marriage – by the use of invalid witnesses and by omitting G‑d’s name from the berachos – in order that they not have a problem of adultery. They simple don’t understand the significance of Kiddushin i.e., a life of sanctification. I discuss this in greater detail elsewhere. It is also a good idea that important rabbis should not be involved in these cases to avoid implying that marriage in these cases is permitted.