This might sting a little. Or, a LOT. It really depends on what you expected from someone, or what you were promised by them ("Oh, I'm totally gonna leave my girlfriend/boyfriend... Just give me time...!")
... And, of course, how strongly you feel about them. It's not an ideal situation, and I know you never planned on getting into one like this. I mean, who has ever fantasised about being the other woman, really? (Unless you're freaky as fuck!)
What I mean is, he's been in this relationship for 'X' amount of time. Granted, long enough time for him to start going behind his partner's back... That would indicate it's fairly long/established. So that would suggest that regardless of what's going on between the two of you, he actually has bought into his relationship's structure and all its predictability. It's hard for someone to walk away from something so definitive in their life...
It could be a case where he genuinely doesn't want to be with his girlfriend, but there's an element of guilt involved with him. You could have experienced it yourself at some stage, where you've stayed with someone just because of the imagined guilt you'd have to deal with when you finished it. Not healthy, not fair, always happening.
But really, besides all that, feeling 'guilt' isn't a justification AT ALL for any of what's going on...
3. She's The 'Cake' And You The 'Eatin''
And he wants to have both of you, duh. The best of both worlds i.e. single life with relationship privileges (cuddles/sex/companionship/advice/friendship/fun/comfort, etc.) and the only time you really wanna be associated with eatin', is when you're balls deep in a Maltesers share-bag (that won't be shared).
4. "Now Is Not The Best Time..."
Hello, reader. Little hint: THERE WILL NEVER BE A 'BEST' TIME TO END SOMETHING. It's always gonna be messy, and hard and awkward. He either wants you and only you, or he doesn't...
5. He's A Bad Person
He (like most people I've met) enjoys the act of happy-sexy time. And, OH! What is this?! TWO girls he can tap up at (nearly) any time he chooses! Hurray for guy!
"No"... "That's not right", you protest... "He loves me"... And only you can see it. "He's not a bad guy, this one. He's just confused... I mean, I can wait another six months, can't I?"
6. Just Because He Says He's Unhappy...
Like, you can't be sure... Hell, he'll tell you anything to get what he wants. Some guys are really good at this. And anyways, unhappy or not, THIS IS ALL BAD! ALL OF THIS SITUATION!
7. HONESTLY THOUGH... Even If He Did Finish It...
... You've heard this one a lot before, but for good reason: Wouldn't he just do the same to you?! Mmm Hmm...