A hell of a lot of men either submit to or agree to unwanted sexual activity as well. I agree that there's more male-on-female sexual assault than female-on-male (partly because the physically-forcible variety of assault usually requires superior strength). But in terms of sex that one partner agrees to despite not really wanting it, I don't think the real numbers are as overwhelmingly skewed as some might think.
The thing is, a lot of men have an internalised sense of "men always want sex, I'm a man, if I turn down sex there's something wrong with me". DH's friend went through a rough patch with his wife and, because they were arguing a lot, he didn't feel up for it. His lack of sexual interest escalated the arguments and he ended up forcing himself to initiate sex because otherwise she would accuse him of cheating and/or not finding her attractive any more. The difference is that, while a woman might process that as feeling pressured or even coerced into sex, DH's friend processed it as "there's something wrong with me, why don't I want sex, it's my problem and I need to fix it". A lot of men, I'd guess most men, would process it that way.
I'm not saying which is the right way of processing it. I think relationship dynamics are too complex for a black and white answer. But there is a definite disparity between how a woman and a man are encouraged to view their choices (or lack of choice) in sexual scenarios.
If a woman gropes a man, the assumption in many quarters is that it's his lucky day. If a girl sends an unsolicited nude to a boy, the assumption (both on this thread and often IRL) is he'd be thrilled to receive it and probably asked her for it. The result is that most men and boys don't report it when they feel uncomfortable or coerced into a sexual situation.