There’s nothing wrong with dating former colleagues. You don’t work together so it’s all fine. But the issue is that she has a boyfriend. Why would you pursue someone who’s not available?

If you think it’s weird that a woman in a relationship keeps meeting up with you, then why are you asking her for meetings and pursuing her? There are plenty single unattached women you can go for. Either just continue friendship and professional relationship or just stop it all together. You think it’s weird yet keep initiating it