Theo wasn't in any position to think rationally or stop the flood of pleasure coming his way, the heat that Greg was stoking up in him... and even if he had been thinking straight, why should he have?
He had nothing. He was living a joint-the-dots life leading to a place he would either die denying or wish he'd die by complying... the not-father was just as distant and cold as his real father was...
Why shouldn't he do this because he wanted to, take pleasure and give pleasure (because he was obviously giving pleasure doing this) and all for himself? Well, even now not even just for himself, but still, the idea was there.
He didn't stop anything. He went with Greg, moaned quietly against the kiss as he felt bare skin grope him for the first time in a long time. Greg was firm with him, wasn't afraid that he'd bruise at the slightest touch, and Theo was pleased to find something to ground him to the real world for once.