Personally I don't consider "futa x female" to be either lesbian sex
or straight sex. However, that's not relevant to the fact that this probably shouldn't be tagged "lesbian".
In order for a tagging system to be useful, tags must have information content, and therefore must exclude some things and include other things. The division between included and excluded classes of content for a tag should ideally be based around what people who use the tags to search for content actually expect to find when they use them in their search — and I don't believe the majority of people who search for "lesbian" on a porn-indexing site are interested in finding futanari content.
In particular:
but the people involved decide what it's qualified as
The only people involved in
the tagging system are F95Zone admins, posters, and readers. The fictional characters depicted don't get a say. (The author does, if they're also an F95Zone poster, but
You must be registered to see the links
ultimately applies — in the end, if the author's definition is at odds with the community's consensus, the community should win out.)