Honestly, I think he wanted clout. I like Brandon Flynn, but I gotta call a spade a spade. Sam Smith more or less implied this when he said that Brandon "got what he wanted from their relationship" and something tells me he wasn't talking about sex. Sam Smith continues to be significantly more famous than Brandon, and as we've already discussed in this thread, Brandon more or less made his road to stardom a even more difficult journey by coming out so early in his career. I'll be honest, I've never seen 13 Reasons Why and didn't even know who Brandon was until he started dating Sam. And I'm willing to bet I'm not the only person who got put on to him this way.
Although I did genuinely know of him through his acting, I do agree with you. I hate to think of Brandon as an opportunist but there’s literally no other reason he would do it.