my sister asked and i'd like to help

So I was chatting with my sister who is in a relationship for almost 6 months now and she told me that she and her boyfriend had a little tiff the other night. Ever since they started having sex, about 3 months into the relationship, she says he always seems to expect it now and seems to pull back or get a little upset when she tells him she doesn't want to.(she said they normally do it a few times a night when they stay together, every time she sees him) Well she told him she didn't want to the other night and he proceeded to ask her why and all that good stuff. She said she just wanted to be with him and she didn't want to have sex every time they were together and he apparently said ok, like he did the one other time she told him no. Then I guess as his way of making her feel bad he pulled the "are you not attracted to me anymore, do you not want to have sex ever again because you don't like me anymore" card. She told him she was very much in love with him and just wanted to spend with him and lately she just didn't feel like his girlfriend. She said that it really had nothing to do with the way he treats her because 99 percent of the time she's treated wonderfully, I wish my boyfriend did half the things hers does. They have their little disagreements but they don't actually fight. She told him that maybe that's why she feels that way. And she also said that he's been saying "I love you" a lot more the past week or so. He's gone from being the guy that says it after sex, or after she says it to saying it quite often apparently. And he bought some sort of book that's like "Things women want men to know" or something to that extent and she worries that that is what got him saying it more rather than him feeling compelled to tell her himself.

I've been noticing that she has been pretty quiet and keeps to herself lately. She asked me if I've ever felt like this and I actually have. I'm not really a confrontational person so I would never tell my boyfriend I didn't feel like his girlfriend. The thing is neither of us really know how to express why exactly that is. So I thought I could come and ask you guys for some advice, I know strangers can't tell me why I feel a certain way. But has anyone ever felt like that before? And does she have any reason to be concerned with him saying "i love you" a lot now? I don't really know why she asks me for advice considering I'm only 20 and in my first relationship and she is 2 years older and much more experienced in that area. Anyways any insight would be wonderful!!! Thanks in advance and sorry for the long post.

[slow_healer]

I'm afraid I have to agree with teko - he sounds like he has some maturing to do. It also sounds like he cares about her A LOT :) so really he probably needs a good buddy to tell him to relax. I'd also add the following pointers:
  Sometimes no really just means no. It doesn't mean "I don't love you" or you're not attractive". Sex can be like chocolate cake; it's delicious, but that doesn't mean you want it ALL the time.
  Sex does not equal love. A strong relationship takes a lot more than a good sex life.
  Eevery relationship has a honeymoon period of intense amazing sex (because it's new and you're getting to know each other) that gradually tapers off into a more realistic, normal relationship.

It sounds like he's getting very insecure - hence the "I love you"s all the time, which may do more to push your sister away than endear her (because it can really come off as needy and desperate). Make sure he somehow gets the message to relax, and if you can, also help your sister with debunking some myths for him. I don't think he's trying to make the situation worse; he's just not working with all the right information about relationships. I hope this helps - good luck!