I feel like there's a lot that plays into this. When I talk about this, I'm going to talk about the romance genre as a whole, not just comics. (And, as always, this is my perception.)
1.) We have centuries of media that's highlighted toxic behavior as romantic. When I finally addressed the fact my ex was abusive, I started to see where a lot of my misconception of love was rooted in romance-centric media. (Especially anime, with the super moody characters who demean their love interests. Ugh.) Partners who were so jealous, they didn't want you to be around exes; dogged pursuit of a love interest, even after rejection; aggression during fights; sexual touches that weren't consented to; unhealthy power dynamics; straight up lying; "I'll die without you, so don't leave me!"
In the end, all the audience sees are these actions being rewarded with a happy ending.
2.) In a lot of mainstream media - books, movies, TV shows, etc - they bank on using reliable tropes, because taking risks means losing money. A lot of romance revolves around some form of toxic behavior: intense jealousy, possessiveness, abduction, control, and so forth. And the way it's displayed is often meant to be positive, so the audience is more prone to internalizing and even romanticizing these behaviors.
3.) Sometimes, writers are lazy. A set of love interests that are toxic toward each other creates their own drama and conflict. You don't necessarily have to write in conflict, if two people who hate each other have to come to terms with falling in love. Writers need to get paid, too, and they get paid more if they churn out more stories.
There's a lot more media with healthy depictions of relationships cropping up. (Or maybe I just had the misfortune to be inundated with toxic stories, growing up.) It's just being criticized loudly - thanks to the Internet, I feel - and we're seeing how bad these things are.
However, the toxic stuff isn't going away. Passengers, a movie starring Chris Pratt and Jennifer Lawrence - is a great example of something really effed up being dressed up as romantic in recent media.
In the end, we need to support media that features healthy relationships and we need to criticize toxic relationships being romanticized and upheld as a norm.