Also, why is there a double standard here? Why can a woman wear absolutely any item of clothing (either male of female) and it's OK, but if a man wears/wants to wear something that the woman thinks of as feminine, he is 'gay' or gender-confused? If a woman goes into a menswear department and buys men's underwear, it is assumed that they are for her husband/BF/SO, but if a man goes into a ladieswear dept and buys women's underwear, it is assumed that it is for him and he must be 'gay' or a cross-dresser - why is that???
I liked Steve K of ComfiLon's comment; if a guy says to you, 'Why are you wearing tights? Are you gay?', just say to him, 'No, I'm not gay; but I know a guy who is, and he has a shirt just like yours!'
Next time your wife or SO wears one of your t-shirts, remind her she is cross-dressing, and ask her is she a lesbian - see how she likes it (and then duck!) :-)