Wow, this is quite a bold and blunt topic. Guess it's not too difficult to see that you like undies very very much (as do I). :wink:
To answer your question, I don't cum in my bikini briefs the way in which you're referring to, but I ALWAYS masturbate while wearing either briefs, bikini briefs, or speedos. With bikinis I pull down the waistband to let it out, with speedos I usually slip my boys through one of the elastic leg openings, and if I'm in fly fronted tighty whities I use the fly front. It's literally been years since I choked the chicken while completely naked from the waist down.
Yeah, I guess you could say that I like tight underwear. :biggrin1: