I usually go partially limp at least once during sex. There are all kinds of reasons. From trying to get the condom out and on. (stupid foil wrappers are a pain to open lol), to getting in position...me and my girlfriend never agree on positions so we wrestle for it
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Dont look at it as a bad thing or so terrible. Like doc said its more fun to get it up again. Also I tend to go pretty soft right before penetration, and my girlfriend loves it, (im above average girth wise so it helps alot if i expand when im in rather than force it in.) Plus have you ever gone from pretty limp to totally solid, while inside? Drives my girlfriend wild, and shes actually came just from feeling me throb inside her