I think he was talking about him dirtying Tamaki with his sexual thoughts? Like, he prefers to take out his lust on others than dirtying the real Tamaki (since he can see he doesn't like sex)
At least this is what I understood
He was a previously closeted and sexually repressed gay kid with a huge crush on his supposedly straight best friend who had just entered a relationship and changed school.
He was just speaking post-coital nonsense, enhanced by his young age and the fact that he felt smug after losing his virginity with a hottie.
But his feelings for Tamaki were obviously genuine all along and never really faded since he decided on his own accord to end the sexual relationship with Takumi, even though he believed Tamaki was straight and in a relationship so nothing would happen between them. Even in the most recent chapter, he denied Takuli's advances twice, despite being horny as fuck.
"A fake Tamaki who'll do it is better than a real one who won't".
WOOOW, just wow. That's awful. To me, that just reads as it doesn't matter if it's not Tamaki as long as I get to have the sex I desire so much.
I know he thought that a while ago and he was younger too, so it definitely could be just his immature self, but it's still a fucked up thing to think if u are supposedly so in love with someone lol