Of course, we've avoided the most obvious reason: you don't have to have spent time around too many dating couples at the office to know how it screws up the team dynamic. Take the bickering and innuendo and breakups and crying, and imagine being stuck with that while sealed inside a huge metal tube for several years.
Dead. Both of them.
Even now, NASA doesn't allow married couples to go on the same trip (there has been one exception, where the couple got married right before launch and it was too late to change the plans). As NASA knows, the true power of love is its ability to make even brilliant, highly skilled people turn into kidnap/murder plotting, 500 mile driving, diaper wearing crazy people.
That simply can't happen on a starship--romantic entanglements would have to be forbidden. You'll have to be compatible with these people, if for no other reason than to make it less awkward when you're drinking their recycled pee.
"So... how 'bout them sports teams?"