I'm sorry, but this has been bothering me for so, so long. I can't find any definite sources. Thanks to those who reply!
Gyne is Latin and (ancient) Greek for woman or female. A gynesexual is someone who is sexually attracted to women. I definitely suggest against using this word if possible because as you can see lots of people will have no idea what you are talking about so it isn't a particularly effective label. Gy in Latin/Greek is pronounced as ji (as in gymnasium, gyges, androgyne) so gynesexual is pronounced jine-seck-shoo-al.
I don't know about Greek, but classical Latin had a hard consonant sound, so G is always the sound in "guy." Plus we say "gynecologist" this way, and IMO at least it sounds nicer. But unless you have a non-binary gender identity, there's little reason to use this word anyway.
Yeah, I wouldn't use the term as my main sexual identity - it just occasionally comes up in conversation with a bunch of my friends and the topic just morphs into a discusssion of pronunciation. So it could be 'guy-nuh-sexual' or 'jine-sexual', am I right? Google Translate says it's 'guyne-sexual' which I honestly can't believe myself to believe. Argent, are you sure about that? Latin pronunciation remains heavily contested in modern times, but in the class I took, I learned to pronounce gymnus (naked) as gym-nuhs. However, gynecologist, which actually has the same root, is indeed pronounced guy. You see why I'm kind of baffled? I imagine it probably is guy-nuh-sexual, but considering it's used almost exclusively by or in reference to non-binary people, and god forbid cishets learn their labels and pronouns, it's mostly not going to matter. Thanks to those of you who responded!
Gynecologist is often pronounced with a j sound everywhere that isn't the US. Gyne is the same word used in androgyne, and I reaaally hope no one is pronouncing it with a hard g there.
It's a term used by genderqueer/non-binary people since 'homosexual' or 'heterosexual' doesn't work if you don't identify as man or woman.
Assuming this is true (which I am) I think that makes it pretty clear. Jine-sexual it is. Thanks again!