I have mixed feelings, but only in as much as, I do feel a bit sorry for the 'in theory wife', but I also am very critical about the institution of marriage and all the social baggage that comes with it, so of course - and not that this is the question - I'm not going to stop doing something that offends a phenomenon that I don't care for.
I don't get turned on by married guys, but when I have sexual fantasies privately, I sometimes fantasise about the idea of having sex with taken men. Taboos get people off. I don't think the thought has ever come to my head when I'm actually doing the dirty though. I don't suppose it does you any harm if it gives you a bit of a frisson during the booking, but I'd be careful maybe to not let it 'get to your head' too much, you know, after you've finished/ start suffering from Mistress syndrome (needing to have these fantasies to feel more attractive than other women/ sexual competitiveness) and all that.