That's odd behavior to me on the male's part. What you said was normal ESPECIALLY because you're a female. I've had it said to me multiple times and I was never annoyed by it by the slightest. Problem with stuttering is that it usually causes other disorders like social anxiety and paranoia and at times lowers a person's self-esteem. You were genuine, but he probably felt it as a jab. You guys are young also. If he were a bit more mature he'd probably would've taken it another way so don't worry about it.