Both girls are not necessarily the worst, but bad enough.
- The childhood friend waited years (big chunks of their lifetimes) until he was in another "relationship". It doesn't really matter what kind of relationship it is. Too bad for her.
- And the dragon mama simply swooned over him until she laid fertilizable eggs. (I don't know how much "being in love with" is needed for that, but this and the fact that she liked him from afar have the potential to be kinda creepy.) Even though this situation isn't her fault, she still does not recognize the fact, that they do not have had a romantic relationship. Not even sex. (Maybe sex is a alien concept for her, as her species does not need it to procreate) And the reason for her to not to is simple. She does not want to recognize it. She wants this relationship to happen. But they are strangers, that befriended each other and now he helps her to raise their children. And he can do so without marrying her.
So in truth, MC has all the freedoms of the world to choose his girlfriend. Both girls may be heartbroken as much as they want, but he simple can choose one of them or even none.