Tell them that they’re making you a little uncomfortable. Knowing when to stop flirting is something that can only be learned by being told to stop enough times that you start to figure out an average stopping point for people. But maybe they think you’re okay with it if you’ve announced to them that you’re bisexual? But given that you identify so minutely as bisexual, I’m guessing you’re not out as a bi guy. Anyway, you should still let them know if they do things that make you uncomfortable and if they’re not outright saying anything you could just ask them if they want to hookup with you when they “insinuate” and then they’ll be given the opening to confirm or deny. That’s when you say speak your peace and let them down nicely.

As far as your underwear goes, keep a closer eye on them. Like when he’s around double check you know where your undies are. If you catch him in the act, end the friendship on the spot because that’s theft and it’s unacceptable.