Complex sentence: There is no specific word in English for a "male mistress", a man in the same relationship to a woman as a mistress is to a man, except for the more general term "lover", which does not carry the same implications.
Split into simpler sentences: There is no specific word in English for a "male mistress", a man in the same relationship to a woman as a mistress is to a man. For male mistress, the more general term "lover" can be used, but does not carry the same implications.