What distinguished the bikini was that it bared the navel. There were lots of two-piece suits before it came along. Contrary to what most people think (including most women), it's easier to wear a bikini with a less than ideal body than most tank suits. If you're conscious of what you perceive to be body flaws, a simple tank is the worst choice. (There are certain tricks that can be done to make the tank more flattering and some companies specialize in these.)
The exposed navel is the key to understanding the bikini's mystique. Like the sundress, it gives a woman an opportunity/excuse to go out in public feeling almost naked. And this is a turn-on for some women. How many? Hard to say. A lot. But it's also the case that a lot of women wear them because they feel they have to. If other women stopped, they'd stop too but peer pressure and need to respond to competition from other women keeps them going. My guess is that far more women wear them because they like it than feel pressured into it but there is no way to measure that.
Covet or curious?Now for some moral theology. Why is it wrong to covet? To covet is not to take. It's just to ... well, to covet. Figuring out exactly what the word means is a bit tricky.
Here's an attempt to improve our grasp on what's wrong with coveting: "to desire wrongfully, inordinately, or without due regard for the rights of others: to covet another's property." The work here is being done with modifiers and, as usual, that doesn't help much. If we can desire wrongfully then it must also be possible to desire rightfully and a bunch of adverbs don't help us to see where the line between the two is.
In moral theology it is sometimes argued that that what is wrong is the kind of desire that obliterates the will of the other person. The last qualification above gets close to this, "without due regard for the rights of others." That's tricky though because to covet is not to steal. I can look at your car in a way that shows no regard for your rights but not steal it because I fear prosecution. "I'd rob you if I thought I could get away with it but I don't think I can."
That's a very powerful argument when we consider sexual assault. There is always a minority of men, indeed, a large minority of men, who would commit sexual assault if they thought they could get away with it. That is something that deserves condemnation; it's a sin.
Or, consider the border issue that isn't assault but is definitely something the woman wouldn't want. A girl in a bikini gets on the diving board and a bunch of guys sitting around the pool start thinking they wouldn't mind if her top came off when she hits the water. It's not assault. It's not even agency—we have no control over what will happen and there is a credible argument that the risk of her bathing suit accidentally coming off is something she is responsible for. Still, for all that we are hoping for something she doesn't want and that she may find humiliating if it does.
That last part is where the sting is. Coveting is wrong even if I never carry through because I have mentally made the move to not caring about the other person.
But we can't stop there. What if some guy sitting at pool side thinks, "I'd like to have sex with her if she were willing"? Leave aside other reasons this might be wrong here because the question is, "What is wrong with coveting" and not what is wrong with, say, adultery.
What makes it super tricky is that we all pretend not to know stuff we really do know. The woman who wears very little on a hot day is not concerned only with comfort. This goes double for the woman who wears very little on a day when it's still a little cool. We live in a world where women openly encourage other people's sexual curiosity. And there is a tacit agreement to all pretend we don't know what's happening when we all know exactly what's going on. As the lemon girl once said to me, "When a man compliments a woman on her shirt, he really means her breasts." The world we live in is one in which some women openly encourage us to be sexually curious about them without acknowledging that is what they are doing. As a consequence, we can never know where she would think is too far.
A lot of sexual morality works like that. We all know what is happening but we tacitly pretend not to know. In that world, which is, after all, the real world, the only world, the world we live in, what are the limits. A woman is dressed in a way that is designed to encourage our sexual curiosity, how far can I let my curiosity lead me before it's too far. I have no trouble thinking of examples that are too far. But I can also think of lots of borderline cases