Missing out on dating?

Say that you've been in a relationship with someone straight out from high school and you're very serious. Nothing was rushed, everything just went into place naturally and you've been together for a year and half. More than likely, things are going into the direction of marriage - or at least being together for a long time. Lately, though, it has come to your attention that your partner feels like s/he's "missing out". They want to see what its like to be with their friends and try and meet new people and kind of just date around. How do you feel about this? Do you feel betrayed? Do you let that person go then take them back with the whole "if you love them let them go and they'll come bacK" thing into practice? Personally, I don't think I could forgive that person. The only reason I could think of someone wanting to date other people is that they want to look for someone else and when they aren't satisfied, you're where they resort to in the end? Fuck that. I guess they didn't love you enough to begin with.

What would you think or do?

Jam it back in, in the dark.