I say it depends on who you are querying. IF you query someone who is pro LGBT it might help get your work infront of them, especially if they are looking specifically for that. But if your book is about an alien invasion/end of the world/world war and this relationship means nothing really and is mentioned once or twice, forget it.
This. Sometimes LGBT is a selling-point and when it is, you should mention it. It's a topic that a lot of agents are very vocal about supporting, so it's easy to find stuff in interviews etc. about it.
But only if the part is big enough that, when the excited agent picks up the manuscript, they're not left going, "what the heck, where is the queer romance?"