R9etb, you raise a fair question, and I'll answer without being flip. The answer is, no, of course not. There is never a legal obligation for a woman to spread her legs, and for a dude to "take" it when it's not otherwise offered is wrong.
That said, the marriage vows created an unshirkable moral obligation on both husband and wife to make sex available regularly (i.e, spread her legs). The reason is simple: If a man or woman is giving up having sex with anyone else in the world forever, they damn well be getting sex at home. For headaches, there is aspirin. For other problems, there is viagra. 'Nuf said.
This dude should have gotten the message: His wife didn't care to have sex with him (apparently - there is probably much to the story we don't know - but we can surmise there were problems). If she was on day 90 of her headache, the solution is to get the hell out because there are affectionate woman who do think enough of their men to provide what they need (and men do need sex and affection - it's not optional).
It's a risk, but a calculated one. One of three things will happen:
1) He will be alone, but better off because at least he isn't putting up with her sh!t.
2) He will hook up with someone who does want him, in which case he will be better off.
3) The wife will be on her knees, literally and figuratively, to get him back, in which case he is better off.
I'm no philosopher, but I speak from experience (see #3 above). Of course, getting the hell out should happen long before children come into the picture, or well after they are grown, but otherwise I stand by my assertions.
< /soapbox >
they damn well better be getting sex at home
Sheesh! I damn well better proofread.