I don’t understand the inherent immodesty of exposing one’s stomach when wearing a two-piece as opposed to wearing a one-piece that may more easily expose more of the bottom, legs, cleavage - the parts that really cause lust, whereas a stomach doesn’t.
I never choose a two-piece that comes off anywhere. I like to actually swim. I usually shop for my two-pieces among ‘granny bikinis’ - the old-fashioned ones with no strings, which offer enough coverage (breasts, legs, bottom), comfort and safety, while not covering your stomach or back.
It’s possible, at least in my country, to get bikinis whose tops are like old-fashioned bras - sturdy straps, with clasps, no strings.
I like to feel the sun and the water on my tummy and back. I hate wet fabric on my tummy.
I don’t think you can generalize like that.
My two-pieces are very comfortable. I like my beaches as deserted as possible, empty ones are the best. After a baby, my stomach is far from perfect, but I don’t like wearing a two-piece any less. Where I go to the seaside, all the grannies, wrinkled or obese or with stretchmarks or cellulite, wear two-pieces, although they’re quite obviously not trying to excite lust - it’s just normal beachwear and no one looks or minds. This is Europe I’m talking about, though. These things ARE cultural.
If an item of clothing (e.g. a bikini) is deemed immodest and likely to excite lust in a country/culture, then it will - it’s as simple as that.
I’d follow the cultural precepts of where you’re living. If you’re in Saudi Arabia, swim in a burquini if you don’t want to excite lust - and be sure to cover the hair, because that’s a source of temptation. If you’re in an African tribe, you could even go topless and no one would get excited. In many parts of the US, it appears, a bikini might be too revealing.