Haha, the guy was basically just a plot device to get two formerly straight girls together. It was a bit unusual how okay he was with the whole situation when he ostensibly had a crush on Momoko. It's not like that never happens, but it's rare. He was pretty supportive of them, too, even when they themselves weren't sure what they wanted to do.
Maybe he's just really into yuri, like us? Sakura assumed he had a crush because he was always looking at Momoko, but most of the examples we see of that are when Momoko and Sakura are talking. He just goes along with it and pretends it was a crush because that's easier to explain than "I was thinking about how cute it would be if you two were dating". Sakura tries to push him into action by asking Momoko out, but doesn't realise his dumbfounded expression was just him desperately trying not to show how thrilled he was to actually see one of his favourite pairings come to life right in front of him. After that, he can get away with encouraging them a little bit, because at that point he's just being supportive of his friend! Even when she says "I was never into her like that, I've actually been in love with you the whole time", he can "supportively" nudge her back towards Momoko and it doesn't mean he's a weirdo who spaces out while shipping his friends or anything! He's just supportive! That's all!