Traditionally, kids have absorbed an understanding of heterosexual love through the relationships they see in real life, on TV, in books, etc., and I think that most people expressing the "same-gender love is adult" view are raising their kids in this environment. In that case, the idea of same-gender love requires (or at least suggests) some type of conversation with the child about some boys wanting to marry boys and some girls wanting to marry girls (wanting to marry was the language my daughter best understood when she was 3 or whenever we had the convo, although I'd prefer other language ordinarily). I have some sympathy for parents who want to have that conversation with their kids on their own and on their own timeline, although that's well short of "adult," and needs to be set at some level where the rest of society can go on acknowledging the existence of LGBTQ people and relationships.