Bad idea. There are other concerns beyond just transmission of STD's, and, as someone said, it's a bad habit to get into. If you put it as something you will just never do, it is slightly easier to avoid in a moment of desperation. It's a very troublesome thing, needle-sharing. I've seen a bit more of it than I would like to admit, although I've never done it ... worst I've done is go after somebody but after cleaning the syringe and even that felt like a boundary violation of an absolute commitment that I've made to myself. As far as sharing rigs with your SO goes, I've known more than 1 junkie couple who did it eiher as a "sign of trust" or because "fuck it, we have unprotected sex anyway" ... but really more often because they got off on it on some levle ... but no reason is really good enough. Rigs are cheap and easy to get (even if you live in a state that is fascist about it, google is your friend), and there's totally no benefit from sharing it ... none at all ... and many reasons not to. Don't do it.