I ask all my partners to
test, and no one has ever reacted badly. Are you offering to
test at the same time? Are you saying that its possible you have something and want to make sure you don't pass something unwittingly to them?
If you are asking nicely, so to speak, and they get insulted, maybe cut them a little break. A lot of people still think "std = dirty" or **** or whatever. If they refuse, they don't respect themselves enough, and don't respect you enough.