Topless is not the usual in St Thomas. I can't remember ever seeing any of this at Maegens Bay, Coki or Saphire which are the main beaches. Perhaps if you ferry over to St John you can find a more secluded stretch of beach where this would not be minded.
If I recall, I think that topless is illegal in the USVI. You might want to check www.vinow.com for that. (there's also a message board there) We've vacationed on St John before (and cruised there) - and there are definitely secluded beaches if you're determined.
"Nude bathing is ILLEGAL in the Virgin Islands. Salomon used to a beach where nude bathers were causally looked over however this has changed. National Park rangers patrol the beach and issue tickets to offenders!"