I agree about the irrelevancy of the sex, really.
Your problem isn't sex, it's having a husband who is able and willing to cheat. It really is that simple, and bringing it back to sex is kind of looking at it from the wrong angle. It's all in this sentence, for me:
'I don't want to risk my marriage.'
Eh? But he is sleeping with someone else. So, what's his definition of this 'marriage'? Because it isn't the same as mine. My marriage is of paramount importance to me - and a fundamental part of what it is is sexual and emotional fidelity. I protect and do not 'risk' my marriage - my marriage, ie what it means to ME in MY head - partly by not sleeping with other people. If I did, I would have chosen to discontinue the marriage as I understood it. There would be nothing of any meaning to 'risk'.
Clearly, he has no understanding of this. No sense of 'marriage' being something within HIM, which exists largely because he continues to uphold its tenets, his promises, and cherishes it as something which chiefly exists in his own heart. No, on those counts he is hollow. What he defines as his marriage is the physical nuts and bolts - 'having' the things that marriage brings like home, hearth, children, security - and your devotion, the whole lot of which he is happy to obtain under false pretences.
So what that one sentence says to me is that this man is a shell. He has little understanding of what marriage is, or should be. Which is why he is likely to expect to be able to 'save' this. He will doubtless say he will 'fight' for you, and 'the marriage'. That he 'will change'. But NONE of that is the point. The point is that what he's demonstrated is that there was never any real marriage, because one of the partners clearly had no idea what it meant. And your knowing that, OP, will not change even after a thousand apologies and years of good behaviour, because he's simply not the man or husband that you thought he was.
I would bet a very, very large sum on this guy, at some point, saying that 'he made a mistake' when you ask him WHY. It's the classic line, and as soon as you think about it for longer than a second, it becomes nonsense. No - you simply chose to have sex with someone else because you wanted to do that more than you wanted to stay faithful. Being faithful therefore isn't important to you. How on earth can he say that to you? - he can't. Hence the nonsensical 'I made a mistake'. No, you made a choice, and a choice that says everything about you.