Ok, I am sure I am going to get nailed to the wall for this. But yes, I do think it's ok to wear a bikini. As long is the bikini is in good taste. No G-strings, no super tiny tops with nothing but strings, ect. I wear a bikini and when there are lots of people starting to come to the pool, I put a T-shirt on to cover up. My bikini is pretty modest. I know to many that is an oxymoron but I feel comfortable and am always aware of what is showing. I am really looking forward to buying the new kind with boy shorts as the bottom instead of the usual bikini bottoms. I feel comfortable in a bikini and would NEVER be exposed without my cover up T-shirt if there were married men and families around. But when there is no one out there by my pool I do wear my bikini. I don't think it's sinful. If I start thinking along those lines I will become scrupulous and that can be the sin of pride. I have so much work to do and really try to watch what I do, say, and think. If I felt or was told it was a sin...I would wear a maxi- cover up dress!
"As stated in the foregoing Article, outward apparel should be consistent with the estate of the person, according to the general custom. Hence it is in itself sinful for a woman to wear man's clothes, or vice versa; especially since this may be a cause of sensuous pleasure; and it is expressly forbidden in the Law (Deuteronomy 22) because the Gentiles used to practice this change of attire for the purpose of idolatrous superstition. Nevertheless this may be done sometimes without sin on account of some necessity, either in order to hide oneself from enemies, or through lack of other clothes, or for some similar motive."
Even St Thomas Aquinas doesn't interpret Deuteronomy 22:5 with strict literalism. He makes exceptions for custom and necessity.
Those commentators who interpret it literally (ie. who say the verse is about clothing) say that its purpose is to keep visible the distinction that God has made between male and female. But this can be done as the customs of the culture choose. St Thomas doesn't identify what these customs must be. The customs may change: all that is needed is for the gender difference to be kept obvious. As it happens, men's trousers cannot be mistaken for women's trousers because of colour schemes and pattern features. A man would never buy women's trousers by mistake. The differences are just too obvious. Or they can be made so to comply with religion. In any case, the free market economy will keep the differences obvious because women's clothing tends to be more costly than men's. If God wants men and women's clothing to be different, so does Mammon.
"on account of some necessity, either in order to hide oneself from enemies, or through lack of other clothes, or for some similar motive." Women can also wear trousers out of necessity. There is no linking pattern between the two specific examples given - hiding from enemies and lack of clothing - to suggest that other reasons for wearing trousers must comply with a pre-set pattern. Necessity is as necessity does. The most obvious is the requirement to be able to do your work safely and efficiently. But so too is the utilitarian aspect of being able to store things on oneself. Skirts aren't renowned for having lots of pockets. And, as has been mentioned before, there also needs to be the ability to move quickly with dignity if an emergency occurs. Generally, a casually dressed man (standard shirt, coat, long pants, ordinary shoes) is dressed to be ready for anything. Not so for a woman. It seems to be cultural quirks that keep women in a state of helplessness (or need-to-be-helpedness) when something happens - tight skirts and shoes with awkward heels that stops you from running, open-toed shoes or slippers that allows broken glass to get into your feet, valuables stored in a clutch or purse that can be dropped, etc.
Why doesn't St Thomas interpret Deuteronomy 22:5 with strict literalism? Because of natural revelation, perhaps, natural revelation being the sorts of commonsense practical things one should be able to deduce if one uses the brain God gave one in order to make good prudential judgements about one's necessities?
For God, a thousand years is like a day and a day is like a thousand years. So, why is the fact that women have been wearing skirts since Eve was a girl of any relevance whatsoever? That longevity of usage is, in the thousand-years-is-like-a-day context, just a coincidence which can be changed as long as the underlying principle that the sexes should be differentiated is kept.
Bring back the Mao. Buttons on the left for women, of course.
When the public swimming pools reopen for business the day after The Parousia, well, that's the kind of garb all good Christian girls (well, all girls actually) will be wearing.