The Drink That Changed Everything

So here's the story: I'm a barista at this cafe, been dating Alex for a year now. We have a great relationship, good communication, mutual respect. But last month was different, stress from work got to me and I started feeling neglected somehow. One night after closing, ended up chatting with Tom, another regular customer. Drank too much, started talking dirty. Next thing I know, we're kissing, it's like a dam breaking. We stayed friends, but it didn't stay just talk, happened a few times since. Why did this happen? Was it really about Alex being unavailable or me being unfaithful for fun? Thoughts?