Interesting NUTRITIONAL INFORMATION about SEMEN.
Since it's full of proteins, vitamins and minerals, there shouldn't be much problem asking your mate to swallow your semen... just show them the NUTRITIONAL FACTS, and paste them on your manhood.
After reading those interesting and funny information, it just gave me some ideas.
1. In order to persuade more of your mate to swallow your semen, go vegetarian, and eat more fruits, at least a few days before your performance.
(since that'll improve the taste of your semen, perhaps eat more strawberries... haha)
and stop taking meat, or diary products since it'll make your semen taste bitter...