There is a story, a proverb that goes around the Hollywood circles that goes something like this.
The first time you have sex with someone, get a flower pot and toss a pebble into the pot. Each time you have sex during the next six months, throw another pebble into the pot. After six month, each time you have sex, take a pebble out of the pot. Conclusion: you will never empty the pot.
That is probably accurate in many relationships, (marriage, living together, friends with benefits, hook ups), whatever you call yourself and another person.
Maybe what they are doing isn’t working so well. Maybe if your relationships are like that you are ready to look into the little book in the Bible called Song of Solomon, or Song of Songs and see if you can’t find a better path for success.