Tristan is very bad for a girl.
It is actually a boys' name, meaning not a surname so the 'surnames are unisex' argument doesn't work. I wouldn't say it is like using
James,
John,
David etc for girls because they have a continued popularity for boys but it is like using
Dominic or
Oscar or something like that on a girl - not overly popular but undoubtedly a male name.
As for sounding 'feminine' because it rhymes with
Kristen -
Maurice and
Doris sound the same in my accent so how about
Maurice on a girl? Or maybe
John on a girl because in some accents (though not mine) it sounds like
Dawn? Or shall I use
Eve on a boy because it rhymes with
Steve? That is another argument that doesn't sit well with me.
For the other part of the post - I don't mind 'feminine-sounding' names on boys mainly because I don't see that as a bad thing.
Sidney,
Darcy,
Aubrey and
Dara feature very highly on my favourites for boys. I don't see it as a problem. I honestly cannot understand why boys must have either very traditional names like
John,
William,
Thomas (though I love these) or uber-masculine names like
Hunter, Cannon etc. Maybe because people are finding names like
Tristan (the name of a knight for heavens sake, how more masculine can you get?) girly?