For that matter, the insistence on permanent mammary glands for everyone to denote gender.
What's wrong with you? Do you expect the actors to cut their breasts off?
Tape and actresses with small breasts. Alexis Bledel or Keira Knightley, for example, could with a bit of artful wardrobe be made to look plausibly breastless. (And incidentally, I owe you a bit of thanks for having to look up Keira Knightley pictures. I forgot what incredible abs she has, or at least had at the time of the photographs.)
In the alternative, you could do it the Shakespearean way.
I personally have no problem with it. Has long has the main characters breasts are kept firmly hidden. Harder to respect someone if you know the actress is there only because of her tits.
If you've got it, flaunt it?

But yeah, the catsuits were pretty immature--and ultimately, I never found the ones in TNG or VOY particularly flattering (gray leotard = sexy?). T'Pol's were moderately better, looked more like a uniform of some type (perhaps a uniform at a stripclub frequented by NASA employees, but still a uniform). It is, I concede, very possible that Jolene Blalock simply had a better body or that
Enterprise had directors that knew how to block better.
The most flattering (female) uniforms they've ever done were the new Trek 11 ones, which were sexy without being OTT about it. Surely, a great improvement over the actual TOS unis, which were apparently designed with patented Upskirt Technology.
Edit: oh, and just to make clear, aliens being analogous to placental mammals isn't that big a deal. I think mammary glands are a great evolutionary boon, especially for a strategy that rewards great parental investment in a few offspring. However, humans are the only mammal off the top of my head that I can think of that carries swollen mammaries around with them
permanently... at a significant cost in back problems, I understand. It's obviously been selected for like a male peacock's feathers--men once chose women with the biggest/most perserverant boobs perhaps as a sign of fertility or health or just randomly, but they've had generations to make children who are likewise attracted to big boobs, creating a runaway effect that no longer bears any relation to the original reason the boobs were selected for (like a peacock's stupid, heavy, "here I am, eat me!" feathers) but has not quite yet run up against a significant selective barrier. The advent of boob jobs may counteract this effect, since obviously silicone isn't transmitted genetically, unless you're a Horta.