When should the hadd punishment for zina be carried out on a man?
Question

                When does a person deserve the hadd punishment for zina? Is it due when the two circumcised parts meet? Is it when there is sexual intimacy without penetration, i.e., when the man emits semen outside the vagina as some people do when they do not want a pregnancy to occur, or is it only incurred when full intercourse takes place?.
Praise be to Allah.The hadd 
    punishment for zina is incurred when the head of the penis enters the 
    vagina. In that case the two circumcised parts have met, i.e., the site of 
    the man’s circumcision and the site of circumcision in the female. If 
    penetration takes place, then this is the zina which incurs the hadd 
    punishment, whether the man ejaculates or not, or if he penetrates to this 
    extent then ejaculates outside, and whether his penis is erect or not. 
It says in
    al-Mawsoo’ah al-Fiqhiyyah (24/23), explaining the conditions of the 
    hadd punishment for zina on which the fuqaha’ are agreed: There is no 
    difference of opinion among the fuqaha’ that in order for the hadd 
    punishment to be incurred, the head of the penis has to disappear into the 
    vagina. If it does not enter it at all or only part of it enters, then there 
    is no hadd punishment because that is not intercourse. There is no 
    stipulation that ejaculation must take place or that the penis must be erect 
    at the time of penetration. The hadd punishment must be carried out whether 
    he ejaculated or not, and whether his penis was erect or not. End quote. 
Secondly: 
This does 
    not mean that a person may be negligent about committing haraam actions so 
    long as they do not reach the level of zina! Rather the intention here is 
    simply to define the zina that incurs the hadd punishment. Being alone with 
    a non-mahram woman, touching her or kissing her are all undoubtedly haraam, 
    and the Muslim must avoid such actions for fear of the punishment of Allaah 
    sooner or later, in this world or in the Hereafter. 
We ask 
    Allaah to guide us and make us pious and chaste. 
And Allaah 
    knows best.