Seven things invalidate wudu:
1. Passing of urine
2. Excretion of stool
3. Passing wind from the rear
4. A sleep, deep enough to restrict sight and hearing. However, if the eyes do not see anything, but the ears can hear, wudu does not become void.
5. Things on account of which a person loses his sensibility, like insanity, intoxication or unconsciousness.
6. Istihadhah (for women)
7. Every state which requires ghusl, such as janabah.
Therefore, if physical contact between a couple is such that none of the above take place, the wudu remains. Fluids that are excreted during foreplay are also ruled as tahir (not najis). Only urine and semen are najis and their excretion will invalidate the wudu.
But if someone has intercourse and enters up to the point of circumcision, regardless of whether the object is a man or woman, or whether the intercourse is vaginal or anal, or whether the two are baligh or not, or whether semen is discharged or not, they will both enter the state of janabah and must perform ghusl, unless they aren't sure whether the male organ has entered up to the circumcision point or not; in this case ghusl will not be obligatory upon them, and consequently, their wudus will still be valid.
It is necessary to point out that janabah takes place through one of two ways:
1. Intercourse, even if no semen is discharged
2. Seminal discharge (even when no intercourse is involved) in sleep or while awake, little or much, with lust or without, and voluntarily or involuntarily.
According to the view of the majority of jurists, the fluid discharged from women that is accompanied with lust (when having an orgasm) is ruled as semen and she has to perform ghusl as a result of it. 
Based on this, it is mandatory for the husband and wife to perform ghusl after intercourse, even if semen is not discharged.
To explain further, the janabah ghusl isn't mandatory in and of itself; but it becomes mandatory for certain acts that require it, such as prayer, fasting, entering a masjid, and touching the writing of the Quran.
For further information, see:
1. Madhy, Wadhy and Wady not being najis, Question 1895 (website: 2719).
2. Performing non-obligatory ghusl instead of wudhu, Question 3249 (website: 3505).
 Tawdih al-Masa’il (annotated by Imam Khomeini), vol. 1, pg. 188.
 Tawdih al-Masa’il (annotated by Imam Khomeini), vol. 1, pg. 210.
 Tawdih al-Masa’il of the maraji’, issue 345; Tawdih al-Masa’il , issue 351; Wahid, Tawdih al-Masa’il , issue 346 and Khamene’I, Ajwibat al-Istifta’aat, the rulings on the janabah ghusl.
 Tawdih al-Masa’il of the maraji’, vol. 1, pg. 208, issue 346.
 For further information, see: Women and Janabah, Question 676 (site: 723).
 Tawdih al-Masa’il (annotated by Imam Khomeini), vol. 1, pg. 214, issue 357.
 Adopted from Question 1711 (site: 1831), Women and the Janbah Ghusl.