They introduce themselves as gay because what if one day they introduce their boyfriend to you?
That would shock you, I know it would, because Gay is not the norm in today's world, it is getting there. Once people accept each others sexuality, then there would be no need to introduce yourself as gay, unless otherwise asked. Instead, they can introduce their boyfriend to you without a inch of astonishment.
They say that because 1, they want to be straightforward right off the start, and 2, they have every right to be proud of who they are.