He didn't wish for Cordelia back to sleep with other women first

Xander licked his lips, then climbed off the bed and approached the two of them. Then he grabbed Cordelia, kissing her passionately.

"I didn't wish for you back to start having sex with other women," he said, sliding his hand down her shorts. With some difficulty. "No panties?"

"They may be important for cheerleading with a skirt," Cordy answered between kisses, "but they'd ruin the lines of shorts this tight."

"Anya, looks like you're the audience, and just to be safe I wish I can't get Cordelia pregnant unless we both want it."

"Granted, master."