The Kings of Cypress & Corundum maybe? They're pretty much the only important male characters I can think of that MIGHT be straight? Any other male characters, it's not clear or really relevant to the story. I think a npc near the spider queen's tower expresses minor interest in her, so impressed with her thread making skills, he was considering asking her to marry him only for Garnet to let him down easy, telling him that he wasn't her type. For female characters, I'm not sure. It's not obvious for many of the not focused on female characters, though most of the ones that receive any amount of focus are gay, bi, pan or queer in some way. Gender is also not obvious or important regarding several npcs. Nonbinary, maybe? For important heterosexual relationships, the only ones I can think of right now are Garnet's parents. Garnet's mom was highly sought after by many before her marriage and her sexuality isn't obvious. Same with the maid Gauche.
A running joke is that all of the monsters look like girls and most of those monster girls are interested in Hinoki and Co and each other, though others aren't concerned regarding the gender of the humans they're after regardless.
The main playable characters are all female, and are lesbians as far as I know. Also there is incest between certain playable princesses that are cousins. This is tagged in the content warnings.