I'll bite - seeing as I have friends who follow the Muslim faith. And ironically, I seem to attract attention from a fair few Muslim men (in Britain) whom are more than aware I'm of no religious faith. Don't ask me why because I don't know why - I'm far from some beauty.
I'd not blink twice as I don't typically pay attention, but, if you asked me to think about it - I'd be surprised, honestly - for a couple of reasons. Bear with me.
For starters, I find it hard to imagine how I'd know they were in a relationship, unless I knew the people personally. Muslims, generally don't typically tend to display public affection in quite the same way that non-Muslim people (particularly of the West), do. Without PDA I'd struggle to automatically assume anybody was a couple, regardless what their religious faith was. I'd struggle to assume any woman that a Muslim man was in public with, was his partner, for just the same reason I suppose. It could be family, in laws, whatever, really. Just as it could be with any non-Muslim man.
Secondly, I understand it is haram for a Muslim woman to marry a non-Muslim man - but, I assumed, perhaps wrongly (hopefully someone can answer this for me, as I've never thought to ask my friends this) this would be absolutely the same, vice versa. I would, have at one time, presumed an outwardly practising man, would want to enter in a relationship with a fellow Muslim, probably outwardly practising also, but I've also met plenty of couples where this is not the case, and the woman does not 'outwardly' practise, if that makes sense. So I'd no longer think that.
That's about as far as it would go, really. Because I'm also very aware of plenty of Muslim women in Britain who do not wear hijab. So it's all moot.
I guess that means that if I even thought about it, I'd automatically assume she was of Muslim faith too. Is it haram for a Muslim man to have a relationship with a non Muslim woman?