If I have unprotected sex on the pill, should I still take the ecp?

No ecp needed. Birth control pills protect from unintended pregnancy, not from sexually transmitted infections. Unprotected intercourse while on birth control pills will not result in pregnancy, so there is no emergency contraception needed.
EC pill. If someone is taking pill regularly then they don’t need to take EC pill for unprotected sex . One still needs to thinks about risk of sexually transmitted diseases . .
May not be needed. If you are on a regular estrogen/progesterone birth control pill and are taking it every day and not missing days, it should protect you from pregnancy. The failure (pregnancy) rate is 3-5% per year on the pill. It doesn't protect you from Sexually Transmitted Infections. For this you need a condom. If you have missed pills or not taken them right, use the ECP. Best wishes!

Related Questions

I've been on the pill just for 3 days, I last had my period 2 weeks ago and had unprotected sex last night, will I need emergency contraception?

Yes. If your period was 2 weeks ago, you were almost at the time of ovulation when you started the pills and may not have been able to stop it with just 3 days. To be safe, emergency contraception wold be a prudent move on your part. Read more...
Use backup x 7 days. The evidence show that the pill is effective after 7 day. So until then, you should consider emergency contraception. Se ec.Princeton.Edu. Read more...