Passion, excitement, thrills yes, yes, and so on... All nice things, but all relatively temporary in nature; and volotile. I've had a few intense, terribly passionate relationships in my life - and I don't regret them for a second. But they take a lot out of you, and at some point almost all your energy is consumed (for better and worse ) by the relationship. There's no more left for other pursuits, be they acadmic, career-oriented, family-oriented or other social activities. These kinds of tumultuous relations almost inevitable end the same way.
The pursuit of passion and excitement needn't be at the top pf the priorities list when considering a relationship - all that will come by itself.
I admire women of principle - and i'm inclined to think that you'd be upholding higher standards (and self-respect) by not being the 'other' woman. What I mean is, why should you *not* command the whole, undivided attention of your mate? Why settle to be a side-dish when you're the filet mignon?
(edit - i had initially left out the *not* - it makes much more sense now )