Does illicit drug use promote ity?

It sounds like your acquaintance is having trouble figuring out who is he (this used to be called "finding yourself" once upon a time). As he explores different ways of looking at the world and at himself via the drugs, he's also exploring different ways of being. Could be that he'd benefit from some counseling, but that's up to him if he wants to really delve into his own psyche or if he'd rather just abuse his body to the point where he no longer cares who he is. I believe that most people have the capacity to be biual, and there's nothing bad about that. However, making advances on friends he knows to be straight indicates he's having problems with judgment (also evidenced by the drug use). So, the drugs per se are a symptom of what's going on with him, not the cause. They're just allowing him a facade to hide behind as he expresses his own inner feelings. I realize it's not your place to intervene, but the drugs are most likely going to destroy him; hopefully he won't take anyone else down with him.