All clothing is only costume anyway , there is no absolute model of masculinity or femininity which says that men should wear a certain item of clothing, in a certain colour and likewise of course for women...in one culture you have the paragon of butch as being a grass skirt or a kilt, in another this is sissy.in one century you have it as being obsence for women to wear trousers.
The reasons for why some heterosexual men find comfort,arousal or even just normality in dressing in clothes which are traditionally deemed feminine are manifold.But the fact that you actively seek to wear something precisely because it is exclusively feminine lies at the heart of it
( ie how many transexuals wear womens trousers?? theyre womens clothes afterall???)
Again though , a million different reasons why ,and no reasons at all too.But its your life and theres nothing wrong with it at all.
If it makes you feel fulfilled, if allows you to explore you identity and sexuality or even if it just gives you a laugh then go for it and leave your guilt behind you, thats the only item of apparel which is wrong to wear