Written by Emily Nagoski, Ph.D.
What they find pleasurable changes (often from partner to partner, as well as from reproductive stage to reproductive stage), whom they're attracted to changes (in Lisa Diamond's longitudinal study,1 30% of her participants who identified as lesbian at the start of the study FELL IN LOVE with a man sometime during the ten years of the study), their sexual interest, orgasmicity, tendency to ejaculate or not... everything that's true about your sexuality today, women, is up for grabs. Ten years from now, it could all be different.2
Because... Women's sexuality is more influenced by social factors. Women's sexual functioning appears to be more heavily influenced by social factors such as relationship issues, social norms, stress, and reputation, compared to men. The typical stereotype that a man can want sex even when he and his partner are not on speaking terms, while a woman wants sex only when she feels emotionally satisfied, is an an oversimplification, but it captures the global phenomenon.
In contrast, men's sexuality is more influenced by hormones. Women's sexuality, similar (but not identical) to other female great apes' sexuality, is almost completely decoupled from hormonal oscillation. This is a complicated and controversial issue, but the broad conclusion is that women's sexual desire, responsiveness, orgasms, receptivity, and proceptivity don't change in a predictable way across the menstrual cycle or across the lifespan.
I can be a little more specific than that: an individual woman might tend to have a particular oscillation of her sexuality across the menstrual cycle (or she might not), but that oscillation will change across her lifespan, and not necessarily in a way we can predict, and it will not be the same as "most other women." Some women have a peak in interest at ovulation, some have a peak before menstruation, some have a peak during menstruation, and others have a peak just after menstruation.
Put a woman on hormones, and christ only knows what it'll do to her sexual interest.
Men's sexuality, in contrast, changes in comparatively predictable ways with hormones. Testosterone is highest in the morning; sexual interest is highest in the morning. Testosterone is highest in late adolescence; sexual interest is highest in late adolescence (depending how you measure "sexual interest").
Don't imagine that I'm saying men's sexuality is simple or dictated by hormones or uninfluenced by lifestyle, stress, relationship, etc. I once had a chat with a college guy who wanted to know what to do about the erection problems he was having, and it turned out he wasn't really attracted to his partner, but she really wanted him and he felt like he was supposed to want to have sex with every woman he saw. He was surprised to learn that not being attracted to his partner was probably a key to his erection issues. *sigh*
Men's sexuality has many of the same issues as women's sexuality; but globally it's a more stable, more predictable, more homogeneous phenomenon. You get to know a man's sexuality today, and then you have sex with him again ten years from now, a lot will be the same. Get to know a woman's sexuality today, and then you have sex with her again ten years from now... who knows?