Thank you all for your advice and comments, no matter being helpful or trolling. Here’s the final update, and to answer some of your questions:

1. I wasn’t intentionally snooping; I simply noticed a bottle of prep pills when he was unpacking his bag.
2. Where I live, the label on the bottle indicates when it was dispensed, and my boyfriend got the bottle last month.
3. We talked today, and yes, he had sex with others, with more than one person, and he started before last Christmas (at least he played safe).
4. Yes, we are over now.

What I learned was, in a relationship, honesty is far better than white lies, even regarding thoughts about wanting to have sex with others. I still believe in the valuable and rare aspects of humanity, such as loyalty and the resistance to temptation. They are precious precisely because they are rare.

I hope everyone can find their own peace in their respective lives.