I don't personally see how I could continue hobbying if I were in a serious relationship. To me a serious relationship is about way more than sex, and if I like a girl that much, to the point where we have moved in together, there is no reason for me to go outside that relationship for sex. If there was, the relationship would need to be addressed, or it would need to end.
There is a Brazilian saying that goes, "Don't spit up in the air, as it may fall on your nose".
Why can't sex be outsourced?
Why do we as a society place such a reverence on sex within a relationship? This kind of thinking causes all kinds of problems. Sex itself is just two bodies merging. When we add hocus pocus to anything, we box ourselves in with regards to it (we create a dogma surrounding it). Sex is a hunger just like food or water. Humans need it- to what extent individuals need it varies greatly. Inevitably someone is going to lose their libido sooner than the other. Then what? Do you just forbid the other from going elsewhere and that's it? Sadly, that's how most relationships pan out. And that's why we companions exist.

But wouldn't it be nice if you built in an outsourcing clause, so that the affected partner can satisfy herself/himself elsewhere without affecting the relationship?
You said yourself that a serious relationship is about way more than sex. So if that becomes the only issue between you and an otherwise perfect partner, doesn't it seem silly to disintegrate the entire relationship solely because of incompatible libidos? Unless by "addressed" you meant a discussion of some sort of outsourcing...