<human>: Hey there r/relationships...

A little back story: I met this girl online about a week ago. We've been really hitting it off, and I've felt a strong connection brewing. We haven't actually met because she lives about 30 miles away; neither of us have access to a car and I've been extremely busy with school/work. We've been talking a fair bit about sex, as she really seems to enjoy it and of course I do as well, but to a lesser extent. 

Earlier this evening (Wed Feb 5th), she tells me that she's been talking to another guy, and they're going to dinner on Friday. I have no problem with this initially. We've been pretty open with each other until this point, so I ask her if she has any feelings for him, and she says yes. I ask if she'd been thinking about having sex with him, and she says yes. A bit of arguing ensues, but is irrelevant. It all boils down to me saying that it bothers me, and I would end relations with her if she had sex with him. 

She says that she has feelings for me, but also thinks that it wouldn't be a big deal if she had sex with him, because we haven't met in person or known each other for very long. She also says that when we do eventually get together, she would end relations with the other guy. Learning this, I offer to take the bus over to where she lives and take her out, or at least hang out with her. She now is committed to having sex with him, even though I've expressed that I'm upset by it. 

I can't figure out whether or not I'm over-reacting to this. It's a big deal to me if she sleeps with him, but she seems unfazed by me being upset. I really do like this girl, and I would like things to work out. I also have concerns that if she does have sex with him, I might feel resentment towards her later on in life if things do work out.

Am I being too harsh on her? Should I not be so upset by this?

Give me the gist of the text in a nutshell
<bot>: A girl I met online a week ago wants to have sex with another guy, I'm upset by this. What do I do?