<human>: Can you condense the text into three sentences or less?

I have been FWB with Nancy for a few months. After meeting through some grad school connections we started engaging in casual sex. Right now we end up meeting about twice a week alternating locations. 

Recently as we were texting arrangements she said that "you are lucky to have me." This made me feel really insecure, like she was just pity-fucking me this whole time. I told her that she was really putting me down but she played it off like some societal commentary. Her supposed point was that the concept of no-strings sex was unheard of for most of history and I should be grateful that I live in a time when it is acceptable. After this argument and got a little agitated that she didn't want to have the conversation any more.

This conversation didn't ruin my life or anything but I do feel somewhat insecure as a result of her statements. She definitely is the "best" person that I've been involved with, sexually or otherwise, so I do feel like a loser often.

Is it acceptable to tell a partner that they are lucky to have you? I feel like this is a no-no but since the incident with Nancy she seemed so nonchalant about it that I don't know if I'm overreacting or what.
<bot>: My FWB told me that I'm lucky to have her, this seems like a put-down but she says that it isn't. Am I overreacting?