Can a man be sexually active during the period that he is undergoing treatments for prostate cancer?

Maybe. Most treatments for prostate cancer have a high chance of causing impotence, so both the desire and ability to undergo sexual activity is compromised. Having said that, I don't think sex itself will compromise the efficacy of treatment unless they involve using drugs that counteract the treatment (e.g. Androgens).
Sex and postate canc. First, sex is natural and good for you and there is no harm in enjoying sex. Potency has libidinal, neurogenic, and vascular components. Sexual ability before cancer treatment is a guide: many have advanced ED before the dx. Each therapy has risk, and the published information is rarely prospective. Ascribing potency failure looking back is biased. If it worked before, likely will work after.
Yes. I assume you mean external beam radiation, the answer is yes for this.

Related Questions

Will I be able to perform sexually after I finish radiation treatment for prostate cancer?

Usually. Usually after external beam radiation treatments to the prostate, sexual function is preserved although the ejaculate volume will decrease to almost nothing.
Possibly. There is a risk of impotence with any prostate cancer treatment even radiation. Whether external radiation or brachytherapy the nerves are radiated and can over time weaken the nerves and cause erectile dysfunction. This depends on whether there were problems before or not and up to 40 percent chance of having issue with an erection. However drugs and other treatments exist.
Maybe yes, maybe. No. The books say 1/3 to 1/2 "are impotent", but few report potency before treatment. As younger men opt for beam or seeds, and follow up occurs, I am seeing a lot more function than what I read in training. Medications, blood presure, vascular disease, diabetes, age and smoking all play a part. Many continue to enjoy a sexual life long after radiotherapy.

Can sexual transmitted disease cause bladder and prostate cancer?

Maybe Bladder. Human papillomavirus 16 and 18 infection are associated with cervical cancer. Oropharyngeal cancer and anal cancer can also be caused by hpv. A recent meta-analysis (li, et al, j infect dis july 15; 204 (2): 217-223) revealed that infection with hpv 16 may play a role in bladder carcinogenesis, but not conclusive. The role of hpv in prostate cancer is unclear.

Will having radiation seeds implanted for prostate cancer affect my sexual performance?

Yes. One of the early effects of a seed implant of the prostate is the the ejaculate volume will decrease to almost nothing. It typically does not interfere with erections at east early on but with time, there is an increased chance of impotence that is thought to be a radiation effect.
Possibly. The nerves around and in the capsule of the prostate will get radiated and there is a 20 percent chance this will effect your sexual function. As far as treatments available the seed treatment has the least probability of effecting sex function.