Human relationships (and not only the sexual ones) proceed like the sound of a musical instrument, with attack, decay sustain and release parameters – same principle as the ADSR controls on a synth:
If you meet someone at a party and do the bizzo later that night, that’s a rapid attack. If you sleep with your friend that you’ve known for five years and always had a crush on, that’s slow attack. Decay is the measure of how quick the ‘honey moon period’ ends, sometimes it’s straight after the peak of the attack phase, but if you’re lucky it’ll last a little longer.
Once things have plateaued out, you’re in the sustain phase. This can last indefinitely (pipe organ) or be over pretty quick (harpsichord). The amplitude/intensity of the sustain is also key – is your relationship loud or quiet? Is that a good thing or a bad thing?
Likewise, the release phase can happen real quick – like if you get busted having an affair (polyphony), or slowly, if one of you moves away and you email each other for a few months but it just kinda dies out in the end.
A one night stand is like the sound of a snare drum, lifelong monogamy is bagpipes.