The reason given for the issur of beged ishah/beged ish is because it's a "toeivah" before Hashem (Devarim 22:5). The Gemara (Nazir 59a) learns from this that the issur is only for a man to dress up like a woman in order to mingle with women, or for a woman to dress like a man to mingle with men.
However, the Gemara in Makkos (20b) and Shabbos (94a) regarding plucking white hairs from amidst black ones seems to imply that one can violate lo silbash without having intent to mingle with the other gender - only that it's something that the other gender would do in terms of appearance. This also seems to be how the Gemara earlier in Nazir (39a) seems to hold, with regard to men shaving hair from underarms and beis ha'ervah.
My question is twofold. First, how is this contradiction resolved? Second, halacha l'ma'aseh, what do the poskim say about this? Do they pasken like the first Gemara, or do they say one may never dress like the other gender?