Hunger, thirst and anger can reduce quality and quantity of semen
 
"Kshudhitaha kruddha chittashcha madyanhe trushito abalaha |
sthitasya haani shukrasya vaayoho kopamcha vindathi ||"
 
According to ayurveda - The intercourse should not be performed when a person suffers from hunger, thirst and anger. If he indulges in sex with these conditions then his quality of semen decreases and vayu gets vitiated. He also looses sexual energy. Vitiated vayu may cause erectile dysfunction, premature ejaculation and spontaneous loss of semen.

Other Causes for Shukra kshaya or decrease of shukradhatu (semen):
 
"jarayaa chintayaa sukram vyandhibhih karma karshanat kshayam gachchatyanasanaat streenam chati nishevanat"
 
Old age, Mental stress and worries, chronic diseases, under nourishment and excessive indulgence in sex or ejaculation of semen leads to shukra kshaya or reduced quantity of semen.