The risk is not desensitization, but sensitization, and not sex but love and bonding coupled with narrative collapse that is greater danger.
When you’re bonding with a person in emotionally heightened state, neuroplasticity off the charts, new memories forming, anchoring to them, trillions of weights adjusting. That song, that drive, the way they like eggs, everything is heavy with meaning.
You’re fine tuning subtle social patterns on them. Learning them. This is a major feature if you’re starting a life together and hopefully compounds with help of predicative processing so your old man brain is literally running their face through a built in Snapchat filter and the 80 year old is experienced as unassailably the most beautiful person alive, you’ve learned to see them in 1000 different ways, and everything reminds you of them.
If traumatic breakup follows though (lost love should be traumatic), pain of the breakup backpropogates through system, presents itself as triggers, emotional walls, you are anti-patterns that can inhibit bonding with your next partner. Which is why standard advice from friends is to FORGET that ex.
ppl be like "sex with a bunch of people desensitizes you to sex and makes it less special" but ignore the fact we expect ppl to bang the same damn person for 30+ years and this is magically supposed to be immune from that same problem??