This one's simple. Porn is sex written for sex's sake. There is no story. No romance between the characters. They are simply boffing to boff. At the end of the day, Character A could care less if he or she never sees Character B again, and vice versa. It's written for sheer gratification, and nothing more. There's no love...no emotion...no connection...and no story whatsoever. It's just sex.
Going a step beyond porn, there is a strong sexual tie in erotica, but it's about more than just sex. Yes, the sexual relationship is key to the story...BUT!!! There is a story. There are feelings and emotions, and while sex is central to the story, the characters don't spend their entire time in bed screwing each other or other characters.
To that end, in erotica, sex can occur between and among more than just the male and female protagonists. All sorts of fun can be had with all sorts of characters, because in erotica, it's not about falling in love, but about something else. It could be about how a character finds him- or herself through their sexual escapades, or about how someone ventures into the world of BDSM and experiments with multiple Doms until he or she either decides on one or decides to walk away. Or maybe a divorcee starts dating a mysterious man who invites her into a world of debauchery, orgies, and partner swapping. She doesn't fall in love with the guy, but they sure have a lot of fun together, even though their adventures create pressure and conflict with her ex and their children. The story resolution is in how the woman makes the two parts of her life work...or not work, as the case may be. But do you see what I mean? A story is being told, the sex is central to the story, but it's not about falling in love.
At the end, the characters get what's called a Happily For Now (HFN) ending. Either they agree as characters that they're not meant to be together and are fine with that, or the story is left open with regard to whether these characters end up together or not. Or the resolution has nothing to do with the couple at all. The key is that at the end, the characters are content with where things stand, even though they are not "fated" to be together, or at least they aren't fated to be together at this moment.
This is where most "mommy porn" writers reside. By definition, a romance has to end with a Happily Ever After (HEA). The characters who have been getting it on for the duration of the story have to end up being together at the end, or it's not romance. In erotic romance, there is love and emotion and conflict. A story is told both when the characters are in and out of the sheets, and they are out of the sheets a lot as their story unfolds. But when they're in the sheets, it's ON! The bedroom door is wide open, and we, as readers, get to see all the action unfold. All the naughty bits are described in rich detail, and we're invited inside the heads of the characters to experience the pleasure with them, much like in erotica. But where erotica ends with HFN, erotic romance ends with HEA. The story is about these two characters, not a bunch of others, and about how they fall in love with one another. The resolution of the story has to show that they end up together...that they're in love...and that they will live "happily ever after." That's the biggest difference.
As for me, I write erotic romance, both paranormal and contemporary. I love leaving the bedroom door open for my readers to see and experience all the fun my characters are having, and I like going into kinkier aspects of eroticism (and my upcoming Mark Strong Tetralogy includes a fetish that is very exciting and will be a source of sexual tension from almost the word go). In my stories, I've used knife play, BDSM, and fetishes as means to heighten the erotic aspects of my stories, but that does not mean they are porn. But a few people describe what I write as porn, anyway. Hopefully now, those people will see that it's not, because while I do get gritty and naughty and down-home dirty with my characters, their stories reign supreme. My characters are on a journey both within themselves and with the people they love, as well as with their surroundings and friends. And that's what I love about writing: the journey the character embarks on. I just happen to expose their personal intimacies rather than close the bedroom door and keep what they do with their partners private.
Mommy porn? Okay, sure...if you want to call it that. But just make sure you understand that mommy porn is really just another way to say erotic romance. :)
Happy reading and writing!