Disclaimer: I am just saying what I think.
We can never really tell you a definite answer unless we somehow get into his head (obviously). Hence, I am going to give you my opinion on the situation.
Firstly, we don't actually know that you are average; people tend to be harsh on themselves. For example, I have friends who are like 8+/10 but they think they're 3/10 and I just want to roundhouse them all in the face. So, in reality, you may be hot as fk and that is why he was interested. Or you might just be his "type". Secondly, if we were to assume that you are average, then, in my opinion, he might just be thinking "pu$$y is pu$$y". I assume he was looking for a one night stand and that's why he invited you over. For guys, they tend to not care so much about appearance when considering a girl for a one night stands, as long as you are not hideous, as when they are looking for a girl to be in a relationship with because there is no commitment and they never have to look at you again. In addition, you approached him so immediately he knows that you are interested. He might be thinking: "yes! I don't have to work (approach, buy drinks and etc) for sex, this girl is down to clown". By you approaching him, you took away all the stress, anxiety and effort that is usually required for a guy to approach a girl and like they say in physics: electrons tend to take the path of least resistance. You are the path of least resistance.
Lastly, but most importantly, when guys are in the mood for sex, then they tend to become desperate.
Note:
- Do keep in mind that he might also be average but you find him attractive because you have low standards
- Text him to find out the answer. If you do and he blows you off, then you know he was just looking for a one night stand. If he is engaged and you go on a date and etc, then you know he was actually interested.