Hans Billian - Das Mädchen mit der heißen Masche (1972)

Plot Summary (IMDb)
Rolf (Michael Cromer), a sex-advice columnist and a lecher, has affairs with numerous women, but never a permanent relationship. Suzanne, one of his ex-lovers, offers to bet that no woman can get Rolf to propose marriage, a bet her girlfriend Andrea (Sybil Danning) eagerly accepts. But there's a catch: To win the bet, Andrea must get the womanizing Rolf to commit to marriage without first going to bed with him.