Tbf, it would work perfectly fine for a character to be in a gay relationship and still have a kid from a hetero one. Plenty of LGBT throughout history had hetro marriages and produced children before they fully accepted themselves and found a partner.
Would be easy to write a story around an LGBT character who was in a previous relationship, but left because they were unhappy, or their partner died and they were left with their only child, and then they end up in a gay relationship.
Easy peasy. Just takes creativity.