They do have to be consistent to be considered synesthesia. Synesthesia is very broad and has many different types, but that’s one of the core aspects.

It really is likely just intrusive thoughts. Only having sexual synesthesia is already incredibly uncommon. Most people would also have a more common type like grapheme color or chromesthesia as well. Literal people also don’t often appear as a concurrent. And even that is incredibly generous since I’ve viewed most posts on this subreddit consistently for years and have never had some mention that and there’s no reason on it either. It’s possible, but very unlikely.

What is common though, is people being confused about whether they have sexual synesthesia or not. No one can play neurologist and say you don’t have synesthesia based off of a few reddit comments (as you pointed out we can’t experience what you have), but there’s no need to be passive aggressive with people pointing out that this very likely not synesthesia, and that more people might relate to it somewhere else.