Hello Jude! *waves
That message you saw suggests that you need to edit their Sexual Orientation settings, which you can do by clicking on a mirror to Change Sims.
Check the "Romantic Interest" and "Woohoo Interest" (these are not the exact names of the settings but I'm not in game right now), changing them to match their partner's gender.
For a heteronormative couple, for example, set the wife to:
Romantic Interest: Men
Woohoo interest: either "Men" or "Men and Women," but not "Women" only.
Hope this works for you, friend.