How is it possibly a topic that doesn't keep him up at night? Rex tips his head in mild confusion, partially at Peter's own expression and the concept of not worrying needlessly about one's vulnerabilities. It's all very odd, he didn't say anything overtly offensive he's sure.

But then he gets the dare and Peter gets to see the most energy out of Rex so far when he straightens up entirely, adding a few inches to his height which is reduced when he rocks back on his heels.

"Gurk." That odd guttural sound comes out in lieu of words, his hands coming out of his pockets to help him catch his balance and once he does he takes a moment to get his breath. And then in perfect, pokerfaced glumness as usual- "...You'll rue the day you did this to me, Peter Parker."

Which means he is gonna do it.

And then he did it!