FWIW, I don't think he came off as an arsehole on that path. It's not a straightforward case of cheating, in fact, you might not look at it as cheating at all. It's at least a couple of years after you leave Val (because she won't uproot her life and go with you into WitSec - which I appreciated, BTW). MC hooks up with a girl a few times, but then moves on when she "catches feelings". He has no idea if, let alone when, he'll ever see Val again at this point. I could be wrong, but I think Val basically said to him to go live his life and to see new people if it came to it, as she is a realist and knew they might never see each other again, and he's still young, etc. (I could also have completely made up that last bit though, to be fair.)
This does, of course, make an assumption that it's the cheating that you refer to as being arseholery...