There is no real way be romantic about it. Fortunately for you, the fact that you're intelligent and responsible enough to bring the subject up will usually make your girlfriend think highly of you, which will lead to more romance at other times. You come out way ahead in that game (and if you don't, she is REALLY not the girl you want to be dating anyway).
The morning after your first time is a pretty good time for those discussions. If that has already gone by and you've been getting it on for a while, then just pick a day when you're hanging out and things seem happy and comfortable. As others have said, phrase it in terms of "I have this rule about always using condoms until we've both been tested." Saying it that way puts the focus on you, not her. Assuming she's not an idiot and/or a slut, she'll be glad you're careful with every woman you sleep with. Girls don't want STDs any more than you do.