First, there is nothing in the image that indicates that either speaker is of a certain gender

Given that the receiver has a boyfriend, it's safe to assume they are a woman. And also, given the context of the post, that regardless of the receiver's gender, the sender is a man, because they are going for that position, the position of boyfriend.

Although you're correct that it isn't specified, it's nitpicking unless you know what the genders are.

I was pointing out how it can be read as passive aggressive

Ok, but in your very example, and this one as well, it's only passive aggressive if you as the reader choose it to be so. It's a pretty good rule of thumb not to take something as passive aggressive unless there's proof that they meant it that way, because that can easily backfire and you'll end up looking like an ass.

If you have ever had a friend who is a straight guy, you can understand how they perceive social ques differently and have different expectations of communication differently than straight women

This is a round-about way of saying that straight guys' expectations of women are primarily romantic in nature. Which is true for a lot of them, but not all of them. There are straight men out there who are capable of being platonic friends with straight women. And more to the point, the ones who aren't capable of that shouldn't expect women to adhere to their expectations of how that communication should go.

Unfortunately, there isn't a nice way of straight forwardly saying, "Hey, I recognize this romantic advance, but that's not what I am looking for in this relationship/I am already in a committed romantic relationship."

I disagree with you. The only way what she said wasn't nice is if he went in with the expectation of being in a relationship with her, which is again, not her fault.

She didn't tell him to fuck off. She didn't say "I have a bf don't do that", she said, "wow that was nice, we really enjoyed it, love you buddy"

She's clearly trying to keep the door open as a friend, if he wants that.

I think your justification of your argument isn't valid and seems more like a knee-jerk reaction to me saying something slightly against the grain.

If what I said was a knee-jerk reaction, you should look at the several paragraphs you just wrote as a retort to the simple assertion I made that she was being nice, and tell me who's knee-jerking.

And lastly, and finally because it feels really stupid arguing this point in the first place, I'm not going to entertain the notion that she wasn't being nice just because you have a penchant for reading passive aggressive intention between two people that neither of us are ever going to know about beyond the words on the screen.

Occam's razor dictates the simple answer is often the correct one. She was being nice. Reading passive aggressiveness from that is an extra step that you added.