Box Turtle Mating and Other Sexual Situations
Osiris, a male, lurks, hoping for sex!
We humans here at the Ornate Bird Garden may snicker, but sex is a big deal to your box turtle, especially if he's a male. In my experience, females seem fairly indifferent to whether or not they get to have sex, but the males are desperate for it. My little ornate male Osiris can smell the females in their enclosure several yards away, and he patrols his own habitat daily looking for an escape route over to them.
Among captive-bred box turtles, the males reach sexual maturity at 4 or 5 years old, and the females at 6 or 7 years old.  You'll notice that your box turtles have sex on their minds almost the instant they come up from hibernation. First, after emerging from the earth, they will probably want to drink some water, but then they will start looking for each other before showing much interest in food!
In my experience, box turtles are most stimulated to have sex in the late spring after they've come up from hibernation, especially after a rain. So you can help them get in the mood to breed by misting their outdoor habitat with a hose.
Ratio of Males to Females. Never let the male turtles outnumber the females. The males will hound the females into a state of bad health with their sexual demands, plus they will fight each other and sustain injuries. I wouldn’t even put one male with one female because he’ll pester her so much. See Which Turtle Makes a Better Pet - A Male or a Female? for more on the important issue of male-to-female population ratios.
Fighting over Females. Male box turtles will fight each other over the females. The clash-of-the-titans between two males involves shoving and butting at one another. The bigger male will often wrestle the smaller over onto his back whereupon the little guy pulls everything in and closes up. If the bigger male is feeling especially vengeful, he will stand over the smaller one threateningly. When the smaller one puts out his head with which to push down on the ground and get himself turned over, the bigger male will bite him on the face! I’ve seen this happen many times, and always try to break it up before it gets that far.
The Sex Act. The act is hard to miss. Typically the male will rush over to a female and mount her. She will continue to sit there, looking completely normal like any turtle that is basking or resting. Often, if caught at the food dish, she ignores the male and continues to eat through the mating experience!
The male will stand up on his hind-legs and rest his plastron (the lower part of his shell) upon her carapace (the upper part of her shell). Often he will grip her with his hind-legs and wedge a claw between her plastron and carapace so that she can't just close up inside her shell and avoid him.
Among tortoises and large turtles, the male will often roar and groan and make other comical noises through the sex act. A male box turtle might hiss now and then, but otherwise keeps silent. The mating can last several hours! 
Sometimes the female will get impatient with this and move off to do other things, toppling the male over onto his back and dragging him around behind her. The poor little guy, still connected to her, will let himself get dragged around as he tries to complete the sex act.
Interrupted sex. If a male and female get interrupted mating, the male will often have his penis extruded through his vent (and don't be surprised by how weird it looks!). You don't want his penis to get dried out because this can lead to a serious medical condition known as a prolapsed penis.
Put him in a mixing bowl with high sides so he can’t climb out before he’s ready.
In this situation, it wouldn't hurt to pop him into a bowl of water up to his plastron until he can get his penis back inside his tail where it belongs. Make sure it's a mixing bowl with high sides so he can't just climb out before he's ready. (See How to Tell a Male Turtle from a Female Turtle for more about turtle penises.)
Odd Sexual-type Behavior. If you ever see a female mounting a male or one turtle mounting another of the same sex, you may be somewhat startled. But don't worry. It doesn't mean that your turtle is either gay or hopelessly confused about procreation.
Actually, this behavior establishes dominance. Most critters in groups want to rank themselves into some kind of hierarchy, and the more aggressive males and females will each think that he or she is the number-one turtle. So he or she goes around and mounts the others just so they don't forget it. It's a way of throwing around the turtle weight and being king (or queen) of the hill.
This behavior is rare among females. But I once had a medium-sized female ornate box turtle that was extremely aggressive. She would pursue and mount all the males who would sometimes try to fight back by snapping at her forelegs. Finally she drove the males into permanent hiding, whereupon she roamed freely, terrorizing anyone who tried to stick his neck out. I found her a new home just so everyone else could breathe a sigh of relief.
 The Box Turtle Manual by Philippe de Vosjoli, Advanced Vivarian Systems, Inc. 1995, p.17.
 ibid, p.44
The Box Turtle Manual by Philippe de Vosjoli is available on Amazon through this link: