I have to agree with YGirl. Some people cannot be sexually satisfied by a single person, but don't want to be "in love" with anyone but their partner.
I also caution you about the "only if my needs are not being met" part. It's really important to meet your relationship's needs within the relationship, rather than avoiding them by meeting your needs elsewhere instead. I don't know much about swinging and what's considered "acceptable" in terms of fixing your own relationship before starting a new one. One of the things about poly is that if you can't "handle" one relationship properly then you shouldn't be getting into additional relationships. But having a much higher sex drive than your partner, and desiring to satisfy that sex drive with someone else (i.e. you can only be so satisfied by masturbating, sometimes you just need pussy) may not be possible within the relationship, if your partner has a lower sex drive. I think I'm just rambling on at this point.
Bottom line, like YGirl said, you might want to research "swinging" and see if that sounds more like what you're looking. If you don't want to fall in love with other people, then you probably aren't looking for polyamory.
Gralson: my husband (works out of town).
Auto: my girlfriend (lives with her husband Zoffee).
The most dangerous phrase in the English language is "we've always done it this way."