In most countries 18 year old are adults. I think it is everywhere in Europe (where I live).
If you are 18, you have the same rights and duties as any other citizen.
Adults should be able to choose their partner independent of race, gender and also age.
How can you support same sex marriage but be against marriage of a couple with a difference in age?
Do you by chance have a wall with post-its, photos and thread? Do you stalk many people?
With 18 people are adults. They can vote and get voted into the government. They can sign business contracts or join the army. They can decide the future of the state but not their own relationship?