Yosef Karo

“ If he spreads a tallit on a case it's considered a container in a container.
7 One shouldn't have relations at the start of night nor at the end, so that others' voices shouldn't be heard and he comes to think about another woman; rather, in the middle of the night.
8 He should perform relations in awe and fear, as it's written of R' A. that he'd "uncover a handbreadth" and "cover a handbreadth" like a demon was forcing him, meaning in awe and fear as if a demon is forcing him. ”

“ For a man has a small member; when he starves it, he is satisfied; when he satisfies it, he starves. But anyone who doesn't need it, but arouses his craving to fill it up, this is the Yetzer haRa's counsel, and from what is permitted to him it will lead him to what is prohibited for him, and about this our Rabbis o.b.m. said: One who hardens himself intentionally shall be in niduy (outcast) .
2 A man must not drink from one cup and set his eyes on another cup (ed.- i.e. have relations with one woman while thinking of another) , even if both are his wives. ”

“ And so too if his intent is for the benefit of the fetus, since during the last six months it benefits it, causing it to come out purified and lively, this seems good. And if his intention is to make boundaries for himself in order to not have cravings for transgression, since he sees his urges getting the better of him and he's craving that thing. ”