The thing here of course is, if probability would be raised to 30, 40, 50% it would be all but certain to be quickly, but if you want that, you better buy those penis enhancements on the net (not yet implemented in game) or start with hung. Yet, with 1000 players trying to knock a girl up, 1 of them (on average) will need 30 tries and still not be there and still only have 20% on the next try and will be frustrated - the problem of a high enough number of players, evidenced at any game forum where these people will (individually correctly) mention how unlikely that all is.
Maybe a slow rise of one percentage point per missed chance of pregnancy or similar could alleviate it somewhat. You're still not certain to be quick, but you get a little lift-me-up and so after 10 tries already have a 6% chance of "missing" and the decrease of it will only accelerate (next try would already be 30%).