Both spouses should know that there are two things which make ghusl obligatory with regard to intimate acts:
Intercourse, which is when his private part enters the wife's private part, even if he does not ejaculate.
It was narrated from Abu Hurayrah (may Allah be pleased with him) that the Prophet (blessings and peace of Allah 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, 348 - the latter added: Even if he does not ejaculate.
An-Nawawi (may Allah have mercy on him) said:
What this hadeeth means is that the obligation of ghusl does not depend on emission of maniy; rather when the tip of the penis disappears inside the vagina, ghusl becomes obligatory for both the man and the woman. There is no difference of opinion concerning this nowadays; there was a difference of opinion concerning it among some of the Sahaabah and those who came after them, then consensus was formed on what we have mentioned above, and this has been discussed previously.
Sharh Muslim, 4/40, 41
Emission of maniy.
It was narrated from Abu Saeed al-Khudri that the Prophet (blessings and peace of Allah be upon him) said: Water is for water (i.e., ghusl must be done when semen is emitted).
Narrated by Muslim, 343
This water comes out of a man in spurts and it comes out of the woman without spurts, but in both cases it is accompanied by feelings of pleasure, followed by physical exhaustion. Each of them has its own characteristic smell.
What the married woman should know is that the discharges that may come out during intimate activity are of two types:
That which comes out to lubricate the vagina and make intercourse easier; this does not make ghusl obligatory; rather wudoo' is required in this case.
That which comes out after orgasm, when climax is reached, and is followed by physical exhaustion. This is maniy and if it is emitted, ghusl becomes obligatory.
There is a difference between the water of the man and that of the woman. The man's water is white and thick, and the woman's water is yellow and thin.
It was narrated that Umm Salamah (may Allah be pleased with her) said: The Prophet (blessings and peace of Allah be upon him) said: The water of the man is thick and white, and the water of the woman is thin and yellow.
Narrated by Muslim, 311.
This water (maniy) is that which makes ghusl obligatory for both men and women if it is emitted.
And Allah knows best.
Reproduced from Islam QA