I have felt the need to write this for a while.
Having been an active participant in both fat and thin intimacy in the past year, it is very clear to me how much better it is to have sex when both partners are fit, thin, and healthy. Diary Of A Former Fat Girl: Sex And The Scale
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I know it is common for many overweight people to lose their sex drive. That wasn't me. Although our sex life didn't lack in quantity when I was fat, it was certainly very different in terms of quality.
Why Thin is Better Than Fat (from a woman's point of view, of course):
1. Only two things on a woman are really supposed to jiggle and bounce during sex.
2. It's a lot more fun being on top, having your partner looking up at you, and knowing he's absolutely crazy about what he sees.
3. Sex with lights on is a lot more pleasant. (Relates closely to #2, yes.)
4. Changes in position during a session (holy clinical speak) are more spontaneous and effortless.
5. Orgasms are generally more intense. (I have no scientific backing on this, I just write what I know.)
6. When receiving oral pleasure from your partner, it is fun to actually be able to watch your partner's tongue at work.
7. More flexibility just means more fun.
8. Up-against the wall sex, or sex in the shower is actually doable.
9. They don't make naughty schoolgirl costumes in Size 2X. (Was that my outside voice?)
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Maybe you have a different perspective (men?) or more to add, but that's my take on this topic.
It Is Disrespectful and Unsexy To Willingly Become Unattractive To Your Life Partner