What should you do if you catch your teen daughter ...

If you think you can stop it, you're wrong. At least she won't get knocked up.

at the time you should not do anything . Teenage years are about exploration of your sexuality. You can sit down and discuss this issue with your child if you are concerned. Generally you will find out that it is mostly done by curiosity and experimentation which is normal but, at the same time should be used safely just like heteral sexual circumstances 

Well, what would you do if you caught her engaging in heterosexual sex?

Sex is sex, if your values and morals are such that you believe she should wait until marriage; then tell her so. That has nothing to do with whom she has sex, her choice of partner is not really your business.

It sounds to me as though are not so much concerned that she was having sex as you are concerned that she might be a lesbian.

Here is a site which may give you some insight: www.pflag.org. Good luck to both you and your daughter.

We sometimes as parents forget that our valves and moral are not always define by the younger generation as correct . Just as we did with our parents .  Try to understand we could not teach people to feel, act and think as we do . All we could do is teach by example and hopefully they will follow and if they don't it doesn't mean they are wrong . We choose our path thats what makes us different !