The premise of the thread is a bit flawed.
Men's swim attire covers much less of their bodies than women's swim attire. Men's nipples, chests and stomachs aren't considered as overtly sexual as women's, but they're not neutral either. (Inarguable that toned muscles and six-packs are deemed conventionally sexually attractive, no?)
Men's speedos are about as tight and revealing as any of the high-crotched bikini bottoms the OP complains about. It wasn't that long ago that they were the default, and they still are in some countries. Some of the skintight swimwear that serious male swimmers wear hides absolutely nothing - serious female swimmers can wear similar skintight suits without having to worry about similar outlines.
And as many posters have linked throughout this thread, there's a huge range of consumer choice for women these days in terms of swimwear that covers everything to swimwear that barely covers anything.
The latter is a valid choice, too. I'm not a swimmer but in the summer I run in a sports bra and gym shorts because it's much more comfortable than a baggy T-shirt and knee-length shorts. It's a performance and comfort issue, not a sexualisation one.
I guess some more pertinent questions would be: why do so many more women want to wear skimpy bikinis, versus men who want to wear skimpy speedos? The latter have fallen out of fashion because of consumer demand; the former are in fashion because of consumer demand. Women are encouraged to sexualise themselves in terms of visual allure in a way men aren't, but a lot of women actively want to do this.