I well remember the feeling of being young, in love and shackled by one's environmental conscience. (I especially remember the shackles, but more of them some other time.)
Aside from trying out biodegradable or vegan condoms, such as those made by Condomi, there aren't many steps you can take to make your intimate relations that much more eco-friendly.
The condoms are important, though, for the planet's health as well as your own - there are more than enough of us out here already without going forth and multiplying some more.
However, you can use the fledgling stages of your relationship to your environmental advantage. As we all know, activities carried out in groups of more than one (more than two is optional) are more likely to be environmentally responsible, so finding yourself in a newly intimate situation presents all sorts of naughty opportunities to behave environmentally.
Start showering together to save water; don't watch TV or play on the computer when you could be losing yourselves in conversation; turn the lights off and keep them off to preserve the mood; move in together and huddle for warmth rather than cranking up the heating. Turn your seduction into an environmentally sound practice by having a go at growing your own roses.
Oh, and if any dressing up is on the cards, make your own costumes (environmentally-friendly shackles included) from easily recyclable materials.
In fact, keep your costumes simple: assorted fig leaves provide a classic look and are biodegradable to boot.