I'm going to take a controversial position. Informed consent is a moral requirement, and any sex act that happens where there is not informed consent is morally rape, regardless of the legality of whatever you're doing.
What's that? You say that isn't a controversial position at all? I don't think you've been paying close enough attention.
Informed consent means that everyone involved in a particular sexual act is fully aware of what they are agreeing to, understands the relevant risks they are taking, and is fully free to accept or decline. It does not simply mean "everyone's over 18".
When you claim to be STD free and you aren't, you do not have the informed consent of your partners. When you claim to be on birth control and you aren't, you do not have the informed consent of your partners. When you claim to be in a committed relationship with your partner and you're cheating on them, you don't have the informed consent of your partners.
When you try to trick people into performing sex acts using lies, you
make informed consent impossible, and transform whatever you are doing
into a rape, regardless of whether the law acknowledges it as such.
There are a ton of people over the legal age of consent who have received a poor or nonexistent sex education, and regardless of what the law says, those people are not capable of providing informed consent. One of the major problems with legally defining an age of consent is that it sends the message that anyone over that line is fully informed and capable of practicing informed consent.
There are people who argue that the age of consent is the age at which a person should be ready to practice informed consent, and if they aren't, that's their fault. They argue that they have a right to sexually exploit the ignorance of others, and that any blame rests with their victims. These people don't care about protecting the vulnerable, and regardless of the legality of their exploitative behavior, they are still rapists.
Treating the age of consent as a starting gun is a serious problem with our culture, and at its root is the failure to understand and really internalize what informed consent really means and why it's so important.