Welcome to the forum. Responding to the title you chose for your question, before I read anything else: it is probably that nobody in the world ever caught HIV by contact with a contaminated towel. I doubt you are at risk.
Now I have read the question. Congratulations for sticking with safe sex with a known infected partner. You had an entirely safe sexual experience. In addition to my comment above, I will point out that the household members of HIV infected persons never become infected (assuming they aren't also sex or needle-sharing partners) despite years of sharing bathrooms, towels, kitchens, eating utensils, and the like. Your situation is obviously different, since your partner apparently is known to have HIV; and I can imagine a theoretical scenario in which semen on a towel could be massaged into one's own urethral opening. But there is no risk at all through contact with intact skin of the penis (or any other body part). As for kissing, that has never been known to transmit HIV, regardless of gum imflammation, sores in the mouth, etc.
As another potential point of reassurance, you could speak with your partner about whether he is on treatment for his HIV infection, and his viral load (the concentration of HIV in his blood). People on treatment with low viral loads don't transmit the infection even with entirely unprotected sex.
Bottom line: no risk, no need for testing, and if you have a regular partner, you can safely continue unprotected sex with no chance of transmitting HIV.
Regards-- HHH, MD
IF semen entered your urethra, then there might have been some risk. But nobody ever was known to have acquired HIV from events like this. Every person with diagnosed HIV infection (acquired sexually) has had insertive vaginal, anal, or (very rarely) oral sex. You aren't likely to be the world's first exception!
If you remain concerned, I would advise you to re-read my comment above about speaking with your partner about his HIV status and treatment. And of course you are free to get tested for HIV if you remain concerned despite this common-sense, science-based reassurance.
There's really nothing more I can say. Let's not get into a string of "what if" questions; there is nothing you can think of that would change my opinion or advice.
Is it practically possible for his semen to enter my urethra and cause infection?
Definitely I wiped towel on my urethra and penis
Now I don't know whether urethra is intact skin or not?
It's just that we both wiped our semen from the same towel with a minutes gap..
Now can his semen travel down my urethra and cause infection ?
I m just worried by theoritcal risk you referred in your answer..