Is it okay to stop tithing if my pastor won't discuss contraception in RCIA?
The short answer
Yes, you are within your rights. Catholics must support the Church, but not necessarily a particular part of it that is failing in its duty. You might give that money instead to a Catholic organization that stands firmly against abortion and contraception, and tell your pastor why.
Within your rights
You may withhold money from a priest who refuses to address a pressing moral problem affecting his people.
The obligation to support the Church
A precept of the Church obliges us to support the Church financially (CCC 2043; CIC 222), but that does not require supporting a particular part of the Church we believe is not doing its job.
An alternative
Consider giving the money you would have given for parish teaching to a Catholic organization that takes a strong stand against abortion and contraception, and perhaps tell your pastor. The faithful have the right and sometimes the duty to make their views known to their pastors on matters concerning the good of the Church (CIC 212).