Voidable means it is not void ab inito, it remain valid or has got some legal sanity till petition for its nullity is granted by the court.
Section 13 Speaks about the voidable marriage-
Any marriage solemnize whether before or after the commencement of this Act, shall be voidable and may be annulled by a decree of nullity on any of the following grounds,-

1. Marriage has not been consummated owing to the impotency of the respondent
2. Marriage is in contravention of the condition specified in clause (ii) of section 5
3. The consent of petitioner or consent of the guardian in marriage was obtained by force or by fraud
4. Respondent was at the time of the marriage pregnant by some person other than the petitioner

Impotency -Marriage has not been consummated owing to the importance of the respondent
Companionship is also a main purpose of marriage, and an individual has a liberty to marring like an important, eunuch, procreation of child is the secondary aspect and it can be bridged by way of adoption. A party is impotent if his or her physical or mental condition makes consummation of marriage a practical impossibility, or inability to perform to sexual act.
Singh J . Defined – Consummation is something referred to as vera copula, means erection, intromission (penetration) by the male of the female. Full and complete penetration is an essential ingredient of ordinary and complete inter-course, though the degree of sexual satisfaction obtained by the parties is irrelevant. After the amendment of 1976, in Rajinder v/s Shanti held a person who is capable of consummation of the marriage naturally or after a surgical operation of medical treatment cannot be called important as the consummation of marriage is possible.
Marriage is in contravention of the condition specified in clause (ii) of section 5

1. Incapable of giving valid consent due to unsound of mind
2. Suffering from mental disorder, unfit for marriage or unfit for procreation of children
3. Subject to recurrent attacks of insanity

Exceptions –
No petition for annulling a marriage on the grounds specified in clause © of sub section 1 not be entertain if-

1. Petition is presented after the one year of consent obtain by force
2. The petitioner has with or her full consent lived with the other party
3. Petition on ground (d) of sub section (1) if the petitioner was at the time of marriage ignorant of the fact alleged
   

Originally under the Hindu Marriage Act 1955, it was laid down that- if at the time of marriage one of the parties to marriage was important and continued to be so till the presentation of the petition , the other party could sue for annulment of marriage.
The marriage law Amendment Act 1976- has reworded the cluse-
The marriage has not being consummated on account of impotency of the responded.