Praise be to Allah.
Not all kinds of foreplay are regarded as intercourse; rather intercourse is when the tip of the penis disappears fully into the vagina. If that happens, then intercourse has taken place; if no penetration occurs or if only part of the tip penetrates but not all of it, then this is not intercourse. This is what is indicated by the ahaadeeth:
It was narrated from Abu Hurayrah (may Allaah be pleased with him) that the Prophet (peace and blessings of Allaah be upon him) said: “When a man sits between the four parts (arms and legs of his wife) and has intercourse with her, then ghusl is obligatory.” Narrated by al-Bukhaari, 291; Muslim, 525.
The phrase translated as “[has intercourse with her]” is a metaphor for inserting the penis into the vagina. This was stated by al-Haafiz in al-Fath.
The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “When a man sits between the four parts (arms and legs of his wife) and the one circumcised part touches the other, then ghusl is obligatory.” Narrated by Muslim, 349.
Al-Nawawi said in Sharh Muslim:
With regard to the phrase “and the one circumcised part touches the other, then ghusl is obligatory”: the scholars said: what this means is when your penis disappears into her vagina; it does not mean touching in a real sense. That is because the circumcised part of a woman is above the vagina, and the penis does not touch it during intercourse. The scholars are unanimously agreed that if he places his penis on the circumcised part and does not enter (the vagina), then no ghusl is required, neither for him nor for her.
The fact that what is meant is what we have mentioned and that what is meant by touching is the two parts coming in line with one another is indicated by the other report, “when the two circumcised parts meet” i.e., coming in line with one another.
It says in al-Majmoo’ (2/150):
The obligation of ghusl and all other rulings that have to do with intercourse are subject to the condition that the tip of the penis disappears completely into the vagina, and they are not dependent on more than that. None of the rulings have to with part of the tip disappearing only. End quote.
Al-Haafiz said in al-Fath:
What is meant by touching and meeting is coming in line with one another, which is indicated by a report narrated by al-Tirmidhi. It does not mean touching in a literal sense, because that does not happen when the tip of the penis disappears. End quote.
Al-Shawkaani said: The ahaadeeth mention the words coming in line with one another, meeting, touching, but what is meant by meeting is coming in line with one another.
Al-Qaadi Abu Bakr said: If the tip of the penis disappears into the vagina, then ‘meeting’ has taken place. End quote.
Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) said:
It is well known that the site of circumcision is above the tip. As that is the case, then it cannot touch the site of circumcision of the woman until the tip has penetrated. Hence it is stipulated that for ghusl following intercourse to be obligatory, the tip must disappear. It is narrated in some versions of the hadeeth of ‘Abd-‘Allaah ibn ‘Amr ibn al-‘Aas: “When the two circumcised parts meet and the tip of the penis disappears, then ghusl becomes obligatory.” End quote. Narrate by Ibn Maajah, 611; classed as saheeh by al-Albaani in Saheeh Ibn Maajah.
Majmoo’ Fataawa Ibn ‘Uthaymeen, 11/323
Based on this, what is meant by the one circumcised part touching the other and the meeting of the two circumcised parts is when the site of circumcision of the man comes into line with the site of circumcision of the woman, and that happens when the entire tip of the penis disappears into the vagina. If the tip of the penis disappears into the vagina then intercourse has taken place and ghusl is required.
And Allaah knows best.