Is it safe to have sexual intercourse without protection when a girl is on her period? if blood touches the penis, does the guy need to be concerned?

Yes, STD possible: It doesn't matter that blood touches the penis, the man can still contact an std. Unless the couple is in a monogomous relationship, a condom is recommended. Regarding ability to conceive: a woman can still achieve a pregnancy if she has bleeding as this may represent mittelschmertz or ovulation that results in increased fluid irritating the lining of the uterus and small amt. Bleeding.

Safe from what: Pregnancy and STD's have been documented during every day of a woman's cycle, including before,during and after menses.Condoms used correctly can prevent pregnancy.They can also reduce passage of some STD's, but not by 100%.The blood makes no difference.