Scuba--
"tactful" or not, being honest is the best way to go if this really bothers you. Next time the planning for a dinner out comes up, addressing the issue head on may be the simplest. You can word it the way that is best for you, many good examples in posters answers above.
If they are good friends, they will understand. If not, well there is your answer regarding your "friendship".
I can not understand why restaurants in your area do not do separate checks! I have never dined on one that refused that request. If they did, I would not eat there again.
If they truly will not do separate checks, then you either need to have the conversation, or decide what is this couples friendship worth to you.
I prefer honesty, but then again, if we couldn't afford to cover a few drinks for friends we like to be with, and that affected whether or not we went to dinner, I might re think the friendship, or my budget.
It amazes me that your friends have not noticed you are not drinking and said anything about paying for their drinks separately!
Best of luck to you.