Eh...I'd have to say pretty much the same, although I wouldn't leave him for sure if he were doing mudsex with an online g/f.
But if he was spending more time with this online g/f than with me, I'd probably leave him. After all, if he's ignoring me over this person he's (hopefully) never met IRL, then he obviously wouldn't mind if I just left the picture entirely. At least, that's what I'd assume..

Well, when I used to be RP admin on a MUSH, I had this problem. What I did was inform both MS'ers that their birth control had failed, and they would be parents of septuplets.
Never had a problem since then.

My 'IC' wife is my RL fiancee. I know offMUD, she's married to/about to marry someone else ([Not a MUD, actually Sims Online, but the principle is the same), but I care not a fig. But hey, I trust her. She flirts with people online whether I'm there or not. Not as some attention seeking device, but because it's funny. I have to agree, and I'm not even slightly worried she's about to run off with some unknown guy from Utah. ;)

So, yeah, there's the word of the day. 'Trust'. If you can't trust your RL SO, I think you're gonna have to lock her in the basement or something.