My head's in a bit of a whirl and I'm looking for some advice.
Firstly, something perhaps I shouldn't have done, was read my boyfriend of four years' diary. It turns out that he went on a friends birthday night out back in March, which I knew about, but what I didn't know about was that he "snogged and groped" another man in the gents, and went on to say he misses the single life and doesn't feel so bad about doing it.
My heads a whirl because I feel angry, betrayed, hurt, upset
I don't advocate reading another's personal writings, but what's done is done. And having read that it would seem you have a right to feel betrayed, hurt, upset, and angry. Surprisingly, so does he now. Unless he intentionally left the diary out where you could read it, or has given you permission you also broke his trust. I would suggest you sit down with him and communicate with him. Tell him its come to your attention what he was doing with another guy. Ask him what he thinks that should mean to your relationship. Try not to be judgmental, but don't be afraid to stand up for yourself.
and wondering what I've done wrong for this to happen.
Other than possibly snooping in his private stuff? Nothing. This isn't you its him. Maybe it was just a drunken moment at a friend's party, but still.... Depending on where your relationship is at (you say you've been together for 4 years, but didn't say anything about exclusivity.... still, most people would assume some level of monogamy in a 4 year relationship) you may have an expectation of exclusivity. If he isn't being faithful maybe its time to move on, or maybe he just needs to lay off the alcohol....
I thought we were going strong until I found this out. It also makes me wonder if this isn't just a one off drunken kiss - has it happened more times? Should I ask him about it? On the one hand I don't want to say anything because I like what we've got at the moment,
Are you willing to risk that he may be cheating as well as risking his health (and yours) on unprotected sex with strangers? How long will what you have going last if he starts stepping out with every Tom, Dick, or Harry that catches his eye?
and I'd upset both our families if anything were to happen.
IF, and I say IF, he IS cheating, it will be he who is at fault for any upset to your families. Again talk to him, find out if there IS an issue. But if there is one it doesn't seem to be of your making.
And on the other, I want to ask him why, if it's not the first time, what's wrong with me etc.
There's most likely nothing wrong with you. If he is cheating there is something wrong with him. If this was just a one time, got drunk and got stupid thing, fine..... Chalk it up to a "he doesn't need to be drinking until he learns to keep his head on straight lesson and move on. If he is (or has been for a while) cheating.... maybe its time to kick him to the curb and good riddance to bad rubbish.
Sorry to be a bore but I really don't know what to do. Any advice would be really appreciated!
You aren't being a bore.... Just a person with a potential issue in need of advice.
Just remember, you aren't completely innocent here, apparently having been snooping through his personal things. Not to pry but did you find his diary left out, or did you go snooping for it?