Sayyiduna Umar bin al-Khattab, Uthman bin Affan (may Allah be pleased with them both) and Sayyidah Aishah (may Allah be pleased with her) used to say that when the private part meets the private part, ghusl becomes obligatory.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 779