I think this probably depends on what you are trying to tell us with the scene of him doing the devils tango with another character. Is this side character important to his life? Does he care about this character? How does he treat this other sexual partner? What does that tell us about the character? Why does he choose his eventual love interest instead of the first partner we are introduced to? Why is the author showing us this scene?
When we are introduced to side characters early on, it can make us feel like this character is supposed to have some significance to the book. If they just disappear and you use it as ‘character development’ for someone else, it can be a little confusing about what this character’s role was in the bigger picture.
I think the reason people don’t like this is the ‘confusion’ that a scene like this can cause if its handled incorrectly. Sexual scenes are very intimate and venerable and it might feel odd to share that feeling with a random side character. Readers don’t know what to do with that extra side character or how they were supposed to feel about them once they are gone.
Another reason people might not like this is internalized slut shaming they are not even aware of. Having multiple sexual partners in a short period can make a character feel cheap, slutty, promiscuous, untrustworthy, etc, to readers, even if they are not aware that’s what they are feeling. They just “don’t like it” even if they can’t explain why.
Also, if he stops his relationship with this other partner because he falls in love with this other love interest, that can feel really icky to the reader. It feels a lot like cheating, even if he is unhappy with his current relationship. Falling in love with someone else and eventually leaving your current partner because of that is a bad look, even if its handled realistically.
Especially because that “early love” feeling is a fickle beast of dopamine and can make us question his integrity to this new character. What’s to make us think he wont just tired of this new lover like he did his last one?
How to handle this ultimately comes down to the author. If you need to establish character has had other sexual partners, you need to decide what the best way to portray that while establishing this new love interest is different and thats why we can trust it.
Maybe his past relationships are mentioned in passing, instead of the reader having to experience them first hand. Maybe this side character does play some part in the book later on and cause some tension, playing a more purposeful part in the book. Maybe he wakes up from this sexual experience and his partner is already gone, thus implying what happen, without introducing us to a random side character. Maybe this other side character is integral to the plot of our man ending up with his love interest and thus plays a greater role in the plot. Maybe he is a “promiscuous” character, and that causes some tension, letting the reader feel justified in feeling that way about him.
But ultimately, it is up to the author. Its just important to disconnect yourself from everything you know about your character and see how they are being perceived by the reader. But I think if you handle it in a way thats true to your character, you are probably safe.