You give her what she wants

Caitlin lays flat on her bed. Your hands wander all over her body as she just lays back and laughs. "I want you to enjoy every minute of this. I am so glad you agreed to donate your sperm to me", Caitlin says. You knead her massive breasts in your hands and try to remind yourself why this is supposed to be a chore to you. You tell Caitlin: "You have such a nice body". You stare at her face, which is starting to turn red, "Yeah... you're enjoying this, aren't you?"

"You know you want to fuck me as much as I want to fuck you", she says, "Why should we wait?". With those words, you mount and penetrate her. Caitlin starts moaning in ecstasy. "You know", she says, "I've actually been saving myself for you. I guess you're a really special guy, huh?", and then kisses your face all over, "Mmmm... you smell so sexy...".

The sweet taste of forbidden fruit (you briefly forget you are actually doing this on permission), Caitlin's massive breasts swaying in front of you and knowing she actually seems to want you too pushes you to your limit in no time. You stop due to the "inconvenience" of your impending release, as she tells you: "It's okay. Come inside of me, so I can get pregnant". You do exactly that and you explode inside her.

You pull out and start to look for your clothes, as Caitlin simply repeats "Thank you, thank you, thank you..." You start dressing, thinking how if you had a bucket list, fucking your girlfriend's sister would definitely be on it and you would have crossed that off today. "No problem, glad to be of help", you mumble as you make your way to the door.

"So, how's tomorrow? Same time?", Caitlin ask. "Huh?", you ask her, almost adding "I thought we were done", but you keep it to yourself. "We'll have to do this again tomorrow, dumb ass. The ovulation test is not magic. I might ovulate today, but I might as well ovulate tomorrow, so we'll be doing this two days each month until I'm pregnant".

Still thinking this must be a dream and not really happening, you absent-mindedly reply "Sure, tomorrow same time is fine". You push out of the door, grinning and thinking how your once-in-a-lifetime has just been turned into a twice-in-a-lifetime experience. At the minimum.

That evening, you play back your brief encounter with Caitlin in your head and rub one out, not caring that you're supposed to save your seed for Caitlin tomorrow. Do you even have to care? The less successful she is in getting pregnant, the more chances you will get at her. Either way, life is definitely good.