New perspective

I just got confirmation that my BF left his pregnant gf basically the day after we had our first date. I'm calling it a date but it was more of "I'm going to be hanging out here at this time" and we decided to meet. We also weren't alone. I was out with a friend; he was out with a friend so we all just met up.  I knew that he was expecting twins.
From his text messages, before we had our first "date" there seems to have been a bit of an issue because she felt like he needed to have been doing more to get ready for the birth (twins). There were texts from her saying she wasn't going to move in with him if he didn't have five cases of diapers by a certain time; that he wasn't doing what he needed to do fast enough, etc. Texts from him were saying that he's trying and doing his best. Her response was that it wasn't good enough. 
After our first date (had known him for some years before our date) texts to her from him changed. He would text each day to ask how she was doing but he stopped chit chatting with her and telling her that he loved her. I guess she was still mad because she felt he wasn't doing enough and told him not to contact her unless he had stuff and unless it was about the babies. After about a week she realized he wasn't trying to chase after her to get back into her good graces and that's when she started getting suspicious about what he was up to. She accused him of seeing someone else and told him if she found something out that she would "kill" him. She would rotate between telling him not to contact her to complaining that he wasn't accessible to her anymore and guilting him to coming to see her. She seemed to realize he was pulling away from her. She started offering him sex and of course it made her even more angry and suspicious when he turned her down. Her text was something along the lines of "I'm starting to feel horney and I don't wanna have sex with anyone else because I don't wanna be bothered". 
I gotta say that we didn't commit to each other at this point. We had discussed it and decided not to tell anyone we were seeing each other because we didn't yet know where the relationship was going. We didn't become official until about five weeks after our first date. We discussed her and how to handle telling her. At this point she was due in about a month and a half and since she was pregnant with twins, it was considered high risk eventhough she hadn't actually had any problems. We both decided it'd be best to wait until after the birth. Of course she found out and the texts were angry and yelling at him; calling him names etc. She was obviously hurt. Reading those texts, I felt sympathetic eventhough she was calling me all kinda names too. 
I feel for her because I've been there too. With my first child I was left by my BF too and I remember that pain. Now I found myself on the other side. Just thinking about that time period, we were so happy (new love and all that) but I see through the texts how miserable she was and it makes me feel bad for her. Like seriously and honestly bad for her despite all the fucked up stuff she's done since then. I can see that her actions are as a result of us getting together. 
I get that some look at men in a bad light for leaving a woman when she's pregnant with his baby, let alone two babies. But should a guy just stay for the baby and if he does, how long is he supposed to stick around if he doesn't want to be with the woman anymore?  
UPDATE:
To clarify, she's absolutely not raising the kids alone. They have joint custody and we are very involved with his kids. He was there when they were born and he's continued to be there. He didn't leave his kids, he left his ex so he didn't walk away from his responsibilities. He takes care of his sons like any parent does. He's a good father. Since she was angry at him she tried to push him out of the kids lives and not let him see them or to only see them at her house under her control so he took her to court, got his name put on their birth certificate, got his last name added to their name because of course she refused to give them his name....the court felt differently. He goes to their MD appointments etc just like any good father would. 
In the 22 months we've been together it's been daily. They dated on and off for five years and would go months on a "break". We havent been together as long as they did but we see each  other consistently 
We just had our first round of <a href="https://glowing.com/glow-fertility-program">IVF</a> and he was my rock. With me he is incredibly loving, kind, attentive and patient with all the appointments and treatments and disappointments. Despite how it sounds, he's a good person and doesn't run around cheating or causing problems.  We simply feel in love during a time his relationship with her was ending. 
To the lady that called me a "home wrecker"...they weren't living together, they were already at odd with each other, and he didn't want to be with her and at the time she was saying she wanted him to leave her alone. How does that make me a home wrecker?  We've been caring for my kids and his kids together so Rachel, there's no problem taking care of the twins. We are a family unit not him and his ex. And no I won't be leaving him for any reason. We are in fact getting married👍🏾😊.