The point of no return near the vagina or anus. The penis will be attracted to enter either hole, but until it goes beyond that hole's event horizon, sex proper has not yet begun.
However, not going beyond the event horizon does not imply that sex will last longer, as the erotic and sensual forces upon the penis that close to the event horizon can result in premature ejaculation.
Professor: As one approaches the event horizon it becomes harder and harder to keep from entering the black hole.
To preform the Dick van Dyke, the man must have their partner get on the ground up side-down, so that their ass is in the air.
The partner then, completely naked, enters the room whilst sining the Dick van Dyke Show theme song. He then trips over a nearby ottoman, thus catapulting him into the air. He then attempts to enter into his hole of choice in his partner.
Fred: Hey, you wanna come over and watch the game at my place?
Ted: Nah, I'm going to the store to buy an ottoman so I can Dick van Dyke my girlfriend.