good way to ruin the moment

It sucks he ditched you, but YDI 100% for saying it in that kind of situation.

That just means you caught him off guard, he took you seriously, and it stirred strong emotions in him. I'd be more concerned if he played it off like it was nothing and tried to use the fact that you loved him to continue to get sex. I would take this as a good sign that he takes you seriously and that something like this scared him (rather than no reaction). I agree with the people above that say it was not the best time to spring it on him, though.

Kind of reminds me of American Pie..

He may have just been shocked by what you had said and that caused him to loose his erection, and the fact that he "went soft" most likely would have embarrassed him causing him to want to leave. In hindsight, it probably wasn't the best idea saying I love you for the first time whilst being "frisky", no offence OP.... However you should talk to him and see how he feels! He may have not reacted in the best way, but how he reacted could very well be justified. You'll never know unless you talk to him.

Why is he still your boyfriend? Dump him

Is no one else questioning why it's is under the "Animals" topic?

Now that you know his kill word you can use it to your advantage. I'd be teasing him for weeks on this one.

I personally don't have sex with someone unless I love them. My boyfriend often says I love you 3 or 4 times during intercourse or making out. I can see how it can be a turn off but I personally get more turned on when someone expresses themselves in a such a vulnerable and intimate way. Isn't being yourself the best thing you can do in a relationship?

Would have egged me on :)