There's more planned, but I wanted to start off with the Mane 6 couples in my nextgen!
I know most of them are straight relationships, but few of them are fully hetero.

Here's some blurbs about 'em!

Caramel and Applejack: Questioning and Polysexual respectively. Caramel doesn't really have a full understanding of himself, and Applejack is not interested in cis mares.
Braeburn and Rarity: Allies. They are straight as they come, but they absolutely support the hell out of the LGBTQ+ community, and Rarity ensures that her boutique is a safe space.
Soarin and Rainbow Dash: Pansexual and Bisexual respectively. They also enjoy FWB relationships more, but they've had longer relations in the past before deciding to hook up together permanently.
Big Mac and Fluttershy: Genderfluid and Demisexual respectively, both are pan. Since the Sisterhooves Social, Mac has started to realize more about himself, uses both he/him and she/her pronouns, and sometimes decides to lean more into one than the other. Fluttershy loves her spouse and supports her every step of the way.
Twilight Sparkle and Shooting Star: Demiromantic asexual and bisexual respectively. Twilight's not quick to fall in love, or even if ever. Shootiing Star is the one person she's ever been romantically inclined towards. That attitude also extends to sex. Star has had past relationships and enjoys both mares and stallions.
Pinkie Pie and Pokey Pierce: Pansexual and Polysexual respectively. Pinkie loves everyone no matter gender, and Pokey's attraction stops at cis stallions.