How can a couple conceive without having to worry about contracting HIV/STD?

Dr Theng has given a wonderful answer. Oral sex is generally considered safe but there is a low risk of transmission if there are some open wounds which may not be obvious.

With unprotected oral sex, there is a risk of transmission of STDs, particularly for infections like gonorrhoea and herpes simplex virus (HSV). The throat can harbour the bacteria that causes gonorrhoea and this can spread through unprotected oral sex. For HSV infection, the risk of spread is highest if the person has a history of cold sores and has active blisters or ulcers. The risk of spread is significantly less in the absence of active lesions. Viral warts or HPV infection can spread through oral sex too.