Depends. You shouldn't let a disease stop you from reaching out and being there for a person, but as far as engaging in a romantic/intimate relationship... that's pretty much up to you. You should keep in mind that a lot of the incurable STD's are still very manageable and treatable, and if you use the right protection you can significantly decrease your chances of catching it (though you'll never fully eliminate those chances as long as you choose to engage in sexual activity with the infected partner, but you can decrease those odds all the same).
I personally think it'd be wrong, mostly because it seems like a rather trivial thing when you take into account how easily you can avoid spreading the disease, even if it's making the hard choice to simply never have sex, but if it's real love then sex shouldn't be a priority to begin with. It just seems wrong to reject a perfectly nice person just because they have a disease. That's just my personal opinion on the matter though, and I can understand the point of views of the people who'd disagree with that. Sex has never been important to me so I'd agree to a relationship with a wonderful person even if it meant I could never sleep with them.