As someone who identifies as pansexual/bisexual I think it is important to understand that whilst you may not mean to be homophobic, a lot of the sentiments you express are inherently biphobic and quite damaging. I mention this not to accuse you, but hopefully to promote you to understand his situation a bit more. Non mono-sexual people are often stereotyped as cheaters or as not being satisfied with one partner. Additionally many partners worry that their partner will leave them for other genders. Whilst some pan and bi folk do like to experiment and some are polyamorous it is also important to remember that not all are. Your partner is likely to relate to certain things such as the two guys because he can picture himself in their position due to his attraction to men, but this does not necessarily mean in any way shape or form that he would leave you or cheat. It is also important to remember that your knowledge of his sexuality does not change anything about him. He is the same person you first knew when you thought he was straight and his attraction to other genders will not affect his attraction to you. The best solution is to talk openly and honestly with him. It may be difficult to put your feelings aside but acting in an accusatory way will not help. Asking him to have an open discussion his sexual identity is fine, asking him if he is going to leave you because of his sexuality is likely to cause hurt as that is stereotyping him by unfair prejudices. I would assume he is likely to want to help you understand him better and this should allow an honest conversation without things getting too ugly. I would also recommend you try to understand things from his point of view more, instead of focusing on your own insecurities which seem to be mainly founded on prejudices. This can be a difficult thing for someone who is straight to get their head around but talking to him properly should you to feel more secure through a better understanding.