I don't know if your relationship is doomed or not, but getting bored from having sex with the same person is a cop-out and BS. I've been married 15 years and would jump my wife tonight if I got the chance.

It's got nothing to do with you and everything to do with him. It's obvious that sex, from his perspective, is about him and not about you or you both as a couple. You say he likes to smoke a lot of weed. Not sure what "a lot" would be, but that can cause sexual as well as motivational issues if it's really quite a bit (like several times a day, every day ... basically staying high all the time).

Personally I think he should step up or step out. If it's really bothering you, your needs aren't being met and you're investing more than you're getting back, then it might be time for you to move on any way, regardless of his decision.