The reason why knowing who you fucked while they were in sexual contact is not even coz of std but not to wonder later on if she or he had sex when it went official and exclusive or before that.

She could have chose to not answer since it might be private and she is uncomfortable to share it but she chose to lie for whatever reason and remember this was before he pressured her when he got to know from other sources she lied.

My question to OP would be , can you trust her after this? Lying comes from a place of insecurity and low self esteem and that's not something I find attractive in a woman. It's your call if you want to put more work into this relationship which it probably needs if it were to work but is it worth it for you is the question you have to answer for yourself.