He's so very awkward. Even if you're a guy and he's straight, if you made advances on him, he'd be too flustered and nervous to reject you. With compliments and praise, you may even get him to accept you. Plus, as the comic has shown, he's not entirely against gay sex — at least, not when it's with his own clone. Speaking of sex, we know he's constantly horny, so, in conjuncture with that last point, he'll be up for anything as long as it gets him off (with enough push, that is).
From the show, we know no one really compliments Morty. Not even Rick, who we've seen to care for him, but he plays a distant, tsundere-like role. Morty is an awkward kid who, from what we've been shown, isn't all too popular at school, so if you were to appear before him, praise him for his ingenuity, he would certainly blush. For that reason, he's the closest thing /co/ has to their own version of Shinji. But who will be Morty's Kaworu?