Firstly, you must be certain that the person is aged 16 years or over and mentally capable of consenting. Secondly, you must be sure that the person consents to engage in sexual behaviour freely and voluntarily.
It is illegal to sexually penetrate anyone without their consent, no matter what age they are. If force, threats, intimidation, deceit, fraud, or tricks are used to engage in sexual behaviour with another person, this is not consensual. The safest way to gauge the other person’s consent is to ask and to receive a clear and unhesitant “Yes”. It is often described as “enthusiastic consent”.
Consent can be withdrawn at any time. If this happens to you, you must cease the act immediately to avoid committing an offence.