Not in this case. If you are having sex with your husband’s cousin to get pregnant, in the way in which you have described to us in great detail, you are clearly doing something unethical.
Your husband is into kinky sex, which makes you (and some of us) very uncomfortable. Kinky sex may not be illegal, but if you are not equally interested in kinky sex, then you are ultimately going to feel ill used by participating in it.
Getting pregnant by your husband’s cousin on purpose is a terrible idea. The cousin, if he is the father of the child, should have all the rights and responsibilities due to a father. If you keep the information of your current or future pregnancy from him, you are being unethical and fraudulent towards this young man.
Your child, should you have one, is being put in a terrible situation. He or she will either not know his true identity, or he will be very confused by having 2 fathers, and when he finds out that his 2 fathers were happily engaging in kinky wife swapping sex, he or she will most likely feel embarrassed or even ashamed.
Divorce your husband.
Get away from this family.
Seek psychiatric help.