I think I agree with most of what has been said so far.
In his book "The Bisexual Option" Fritz Klein argues that sexual orientation can change over time. This seems to be at odds with the "born this way" argument. Can they both be right? What I suspect is happenning is selective expression and repression, i.e. someone born with the capacity for attraction to both sexes does have an element of choice over which one he acts on at any given time.
So go back to childhood and we learn that mummies and daddies love each other, get married and have children like us and we start to project ourselves into that future. Then we discover we have at least some level of attraction to both sexes. Finally we get some level of negativity about same sex attraction. Exactly how much depends on when and where you are at the time those feelings become aparent. So that probably means choosing the straight path, i.e. acting on your attraction to women and repressing the attraction to men, at least for the time being. That choice may even be unconcious.
Then, after a good few years, she is not as interested in sex as she was at the start, especially if she has reached the menopause, and our minds start to wander. Apart from the repressed male attraction something else comes into play, summed up by this joke:
What does a lesbian bring on a second date?
A U-haul (for moving in).
What does a gay man bring on a second date?
What second date!
If a woman fancies you she will want a romantic relationship. Sex will hopefully be part of that but she is unlikely to be satisfied with being friends who occasionally have sex. If you start an affair with another woman she will eventually want you to leave your wife so one or other of them will be disappointed and there will be a whole load of stress.
Men, on the other hand, seem much happier to meet for occasional sex whether in the context of a friendship or otherwise. You can meet another man for some fun and just leave it as that. It's simply sharing the male need for release.
WOMAN TO MAN:
You FUCK with me...YOU STUCK with me