update: added the Steipler Whatever is done has to be with mutual consent - Nedarim 20b, Shulchan Aruch 240:3,
Rambam(Hilchos De'os 5:4): ...Neither of them should be drunk or uninterested or depressed nor should either of them be sleeping. The man should not force the woman against her wishes but whatever is done should be with mutual desire and with joy. ...
Steipler (Igros Kodesh #2): [...] The details of sexual intercourse of the mitzva of conjugal obligations are explained in the Siddur of Rav Yaakov Emden in the section of the laws of Shabbos night. It should be studied carefully because these are actual halachic obligations. Because according to the Torah it is prohibited to have intercourse in circumstances where the woman is not interested and therefore it is necessary to get her interested with intimate activities such as hugging and kissing until she is sexually aroused and desires intercourse. Because if this is not done first – it is like placing her before a lion who mauls and eats as is explained in Pesachim (49). [It is a terrible sin to deprive her of her sexual rights even if he is doing it for the sake of piety and asceticism. Taking what belongs to his wife can not be the basis of piety by theft and treating her as a slave]. Furthermore to have sexual intercourse against her will results in having children who are sinners and rejecters of Torah – they are call bnei anusa (children of rape).
A husband who grabs and has intercourse immediately without preliminary intimacy and then separates immediate after ejaculation and leaves her, might think that he is acting like a highly spiritual angel. But in actual truth he has not lowered his lust at all even though he definitely has placated his desires for the time being since he has in fact obtained full pleasure from this intercourse. In contrast his wife felt no pleasures at all from this abrupt sexual act. In fact she has been hurt and shamed and the tears she sheds in private will not go unanswered. That is because our Sages(Bava Metzia) tell us that the Gate of Tears has not been closed. They say that a man should be very careful with his wife's honor because she readily cries. There is no question that such a brutal act arouses Divine judgment against him.
Furthermore he does not merit to have Divine help either in spiritual or material issues. And this that he mistakenly thinks he becomes a highly spiritual person by trying to be insensitive to his wife in sexual matters – this is a worthless fantasy and a lie. That is because from sins and transgression one becomes blemished and spiritually impure – not elevated.
[...]
Igros Kodesh (#4): Question: How to avoid emission of sperm while asleep at night when his wife is a nida and also during the time when she is permitted to him?
Answer: I am forced to answer even though it is very difficult to clarify the matter in a written reply. From his letter I got the impression that he needs guidance in the broader topic of what is the nature of married life. Marital relations need to be such that he is sexually satisfied even during the time when he needs to separate from his wife because of nida. Thus we are not talking about the number of times of intercourse but rather the quality of the sexual relationship.
Young men, especially those who are serious about spiritual development, think that sanctifying oneself during sexual intercourse can only mean that one needs to minimize lust and sexual feelings as much as possible. But this is a complete mistake as can be seen from the enlightening comments of Rashi (Nida 17a). Also please look at the Rokeach (Jerusalem edition page 27a) who cites Nedarim (20b) that everything that a man wants to do with his wife he can do in order that he won’t have any interest in other women.
We see clearly that sexual relations need to be so satisfying until all other women are in his eyes as hens and he absolutely no desire to even look at them. This is also implied by the statement of Chazal concerning the reward for those husband who prolong physical contact with their wives after intercourse is finished. Thus we see the need for intercourse to be intense and satisfying – and not done merely to fulfill one's obligation and similar extrinsic matters. And therefore if a person is concerned for the quality of intercourse itself, I am sure that he will be so satiated and satisfied that lust will not be a problem even during the period of nida.
Aside from this, he needs to strengthen his Torah study. That means not merely study with a book but to train his thoughts to be involved with thoughts of Torah study. For example before he leaves his house or yeshiva to organize Torah issues to think about until he reaches his destination and that he shouldn’t be empty of Torah even when he is walking in the street.
And regarding not looking at women – there is the advice of the Gra that one should pray before leaving to go into the street – that G-d should guard him from all aspects of sin including sinful thoughts until he reaches his destination (See Orchos Chaim #135 at the end of the Gra’s Siddur).
Furthermore he should learn with joy and not worry or fear at all. That is because worry and fear just arouse the mind and nocturnal emssions –G-d forbid. And with G-d’s help if he conducts himself as I have described, he will see success.