Let me address this issues of sex; sexual thoughts, as it is an important issue. Also lets touch upon Venus.
A strong Venus in the birth chart cannot be made weak; it will remain strong; but for good sex two people are needed; so both should have good Venus (and other planets). Hence, the complete chart has to be seen for the individual and / or couple. Also Venus is just not about sex; it is about much more in a persons life.
Sexual thoughts and indulgence in sexual activity is a very normal phenomenon; it is a human need; like we need oxygen. It is also the way by which we procreate.
I would be more worried about the quality of one’s Venus being afflicted; if there is lack in sexual thought or lack of sexual activity/drive or disinterest/failure in it, moreso from puberty onwards; leading to distress in multiple spheres of ones life.
Now due to what reason the quality of ones sex life is not up to the mark; can be known by studying the complete chart. Sometimes it can be due to stress; sometimes it can be due to problems in the reproductive organs; there can be many reasons.
Of course the exception are if you have become a celibate or you have taken sanyas or Sainthood; then even masturbation is a sin. But that doesn’t mean you have a bad Venus. By mental strength and by immense tapasya one learns to preserve the vital forces; that is produced as a result of the stimulated sex organs. The more inward your mind goes living this monastic life; the more can these forces be preserved or controlled automatically. Also you live in a austere, pure, pious environment.
Some people do have excessive sexual drive that does lead to excessive sexual indulgence, which is not good as everything has to be within limits. Keeping our sexual urges in balance and channeling them properly and appropriately is the difference between human beings and animals. We have to live by societies rules and accepted norms and laws.
Significant over indulgence in sex also leads to a lot of energy wastage and quick mental (and maybe physical) decline at an early age.
But I want to mention that in good charts showing a good marriage/relationship – a healthy and a good sex life is seen and is mandatory.