Some boys wear their sister's underwear and later their wife's. If unmarried they have to buy their own women's panties or steal them off washing lines. A guy in my home town even got caught by the police trying to steal panties with a fishing rod through the letterbox of a sex shop at midnight.
I believe the objective when wearing them is to suppress their masculinity by staying soft, even hiding their flaccid bulge. Koala Swim make a thong that takes the penis down and under completely concealing it (while displaying pubic hair) for a feminine appearance. In any sexual experience they have in this mode they assume the woman's role.
Someone with this fetish may well find the tightness of a speedo or smooth front bikini a feminine experience, while to the rest of us it is masculine. It seems best to accept their feelings while holding our own in our own ways.
http://www.koalaswim.com/ko_all_items_master.htm look for T-girl Sheer