Hello Good Evening,
There are nothing to worry, nil chances of getting pregnant by that towel which used to wipe semen.
Fertilization can not be possible through clothes, or without any penetration or actual intercourse.Pregnancy
depends on directly proper ejaculation of semen in the vagina by intercourse and during fertile period of woman.
So be relaxed, in the case of the towel BUT there are no 100% success rate (1-2 % failure rate) of condom, which you have use during next round of actual intercourse.
So you have 1-2% chances of being pregnant, if you were in fertile phase of your menstrual cycle at that time, just because of condom round of your intercourse, not because of towel.
So be watchful for period to come, but be relaxed till then.
Dr. Naresh Jain