Just as in real life sex, you need to both be in agreement and consent for whatever you wish to phone or internet sex about.
So ensure you engage in open and honest communication around what you're up for and where your boundaries are.
If you're monogamous, where does the boundary lie in respect of talk of another person - is that banned or is that ok for you in a fantasy scenario of phone sex or internet sex discussion with your partner?
If there's kinks and fetishes involved, is there a limit to what the other partner is ok with and not ok with?
I would suggest you to see if the other person or your gf is okay with sexting or not. It's not self-love to be dishonest around something you're not ok with.
So, apologize her right now, if you really want to be with her and yes be careful next time!