I have a lot of platonic male friends. I've had some of them sleep at my place, and I've crashed at their places, for reasons varying from a vacation visit, to not wanting to drive home after a few drinks, to needing a place to stay while finding an apartment. I've also had, by quick count, 20 male roommates as an adult, with relationship statuses varying from single to in a relationship to looking for a relationship, to engaged, to married, to long distance relationship, to divorced.
Not once has any of them behaved in anything but a polite and appropriate fashion as far as sex and romance go.
There are some guys I know who I wouldn't share a room or apartment with, because I wouldn't trust them to behave, or there are complicating factors. For example - a guy with a jealous or controlling girlfriend/spouse, or a situation which involved the guy lying or hiding it from his SO, or who had troubles with alcohol. I also wouldn't do it in the case of a guy who I knew had feelings for me, because that's asking for a problem. Sharing a room with a platonic male friend
and sharing a room with someone when there is one sided or mutual attraction are very different things.
So no, all men are not slavering wolves completely unable to resist trying to have sex with any woman they happen to be alone with, whether or not that woman is interested in them.
For my husband, in general, I wouldn't have a problem with him visiting a female friend. If I didn't trust him to behave when I wasn't there, I wouldn't have married him. I would, however, reserve the right to veto a particular instance that made me uncomfortable - the most likely would be a case where the woman was making a play for him, but he didn't realize or notice.
The OP's case, though, is one where she *doesn't* trust her boyfriend to behave properly when visiting his friend, even if she hasn't really admitted that to herself. And from previous posts, it sounds like that mistrust is realistic, not coming out of thin air.
The big thing will be how the BF reacts if she voices her concerns, though.