What did my boyfriend mean by saying this?

When he said "oh it was a good attempt," it can mean different reasons. It can mean that what you described as your fantasy, he might not think that was sexually enough. Or, it could mean that you can think of sexier ways. Just ask him what he meant by that and to see what his response is. If it bothers you, you can either confront him, or just let it go knowing that he really loves you.