At what point does one "settle" for a comfortable marriage?

My marriage of fifteen years is, for the most part, satisfying. My husband is loyal, dependable, helpful (domestically speaking) and he genuinely adores me in his own way. However! He is unimaginative and somewhat rigid. I am overemotional, hypersensitive to what I’m feeling/needing and am more of a free spirit. During the years when our two daughters were young enough to require more attention at home, I was happy to be home. Now that they are 12 and 14 and are developing interests outside the home, I have rediscovered my desire to be more social – I enjoy things like trivia and karaoke (coupled with a few good pints of British ale). My husband would prefer to go to bed at 8:30 every night. Adding into the mix (and it’s a BIG add in), I have recently reconnected with my first love, starting out as just “catching up”, moving into discussing the attraction that is still there and currently into a full-blown affair. My problem is that I am very comfortable with my husband who knows me, accepts me and my moods and we have a good life BUT the creative/emotional side of me is only fulfilled with my first love. I am probably deluding myself that my husband would ever accept a polyamorous situation but I can’t help wondering. Also in the mix, my lover has a long distance relationship with another woman who he also loves very much.