Cuoco, who just married her tennis star boyfriend, Ryan Sweeting, after dating only six months says that it's time for her character to tie the knot wither her scientist boyfriend, after all, they've been dating for six years now! Leonard has asked Penny to marry him on several occasions but it was never the right moment, such as when he asked her after sex, or when he asked her before she broke up with him once. Then, she asked him to marry her once but she was drunk and Leonard didn't feel like it would have been appropriate to say yes because he didn't want to take advantage of her.
The interesting twist in all of this marriage business is that Penny was married at one point, but she didn't know it. She went to Vegas with a boyfriend and they stopped at a little chapel while drunk and were married by Elvis. She didn't think it was a real wedding, therefore, didn't think of it until she found out through the grapevine that they were legally married and needed to get a divorce.
Fans would love to see Penny and Leonard get married, but as Sheldon so eloquently put it, "where would all three of them live?"