It was during his amnesia and he didn't do penetration, because he didn't get an erection with anyone except FML. But he did other things.
You have to know this is similar to guideverse, but unlike typical guideverse, here everyone in the universe is an esper and guide at the same time. Thus their society was open-minded about sex.
Since the FML was a transmigrator from Earth with ordinary morals, she used drugs to eliminate the side-effects of using her powers instead of sex.
While the ML grew up in this universe with such open mindset everywhere.
Not to mention the ML is the strongest in this universe thus the side-effects of using his powers is especially strong. Most drugs would be useless for him. And even the ones with some effects might not be enough.
The FML was Level 2 thus the drugs were effective for her. She was practically at the bottom regarding strength. But even so she became a drug addict.
The ML was Level 11 or 13 I think. He was practically has no choice but to turn to sex.

From what I gathered from korean comments the ML is a promiscuous man who sleeps with both men and women.
Well sex was a necessity because of some physical anomaly in their world or something.
He loved toying with the FML. Even thought their compability was high their level difference was huge which made her terrified of him when they had to do the deed.
The FML was a virgin because instead of sex she used drugs to suppress that physical anomaly.
Naturally the ML used sex unlike the FML before meeting her.
Well he was more than 10 thousand years old so expecting him being a virgin is a fantasy itself.
He even cheated on the female lead even though he was supposed to be devoted.
I think it was about him forgetting the FML like some amnesia trope.
And to fill the the void the FML left inside him, he had orgies with both men and women.
I read that he didn't do penetration then because he could only get an erection from the FML. But anything other than penetration he did it.
So all in all this was the kind of work that those who like it like it and those who don't don't.
Supposedly the novel has high ratings so there were many who liked it.
And those who don't it was because of the above points partly.