Hi! I have a 4 year old male pug. He acts like the 3rd-in-command of our household, second only to me and my husband. For a while, before my husband learned to assert himself as alpha male, the pug acted as though the chain of command was me, then the dog, then my husband. Pugs were bred for 'personality' - which translates to a predilection for dominance and uppetiness. Any dog will try to insert themselves as close to the head of the pack structure as possible, and pugs don't have the genetic predisposition toward submission that spaniels have, for example.

I'm not so sure that your male pug thinks the female is in heat. Is he penetrating, or just mounting and humping? If it's just the mount and hump, you're absolutely right in calling that an attack - mounting behaviors like those are about dominance, not reproduction, and can occur male-on-male, female-on-female, and even female-on-male, when one dog decides it wants to show another who's boss.

Sounds to me like you have a case of a male pug who believes himself to be the alpha male of the household. He's acting up in ways that are aggressive, dominant, and possessive - marking (this is MY den) and mounting (this is MY bitch). (Has he marked your laundry?)
Not good.

I am guessing that there were no other males living in your household before your boyfriend moved in. Is that correct?
If that is the case, then your pug has been the alpha male of your house. No matter how in control you may have been, there were simply no other non-female creature in the pack to assert any kind of dominance over him, so he is not used to having to be submissive to anyone of the male variety.

Am i also correct that this is your first child?
If so, then your dog's previous role in your life is under a double threat.

Still - not acceptable behavior from the dog. Particularly not when there's a child on the way. Good for you that you love your dogs enough that you are not willing to part with them. But it's important that you make your decision based on what's best for YOU and the child. YOU as the alpha female of this pack need a strong and predictable alpha-male mate who can lead the pack with you and help you in the task of taking care of your young. And you need your dogs to be sweet, loving companions to the both of you, obedient and predictable, and respectful and gentle with your child. And your dogs need you to show them leadership and guidance and structure in this time of change and uncertainty.

I do believe in discipline. Having a clear and simple set of expectations and recognizable consequences, both positive, neutral, and negative is absolutely crucial We ALL need to know what is expected of us, and that there are behaviors that are desirable and wanted and rewarded and a couple that are extremely negative and will meet with correction.

If your boyfriend is willing, have him take over some of the provider roles with your male pug - have him feed the dog, and only put the food bowl down after the dog obeys the SIT command from him. Then as they get better with each other, he can start using more difficult commands. That will teach the dog that your boyfriend is a source of food, and that obeying him has positive consequences.

I also strongly recommend that your boyfriend take the dog for walks, or even runs (long jogs are bad for pugs, because of their breathing - but a combination of sprinting then walking can be great, if the dog -and the human!- can handle it). A tired dog is much better behaved. And it will make your boyfriend the provider of another good thing - FUN!

As for discipline to correct the marking and mounting:
If you see your dog starting to lift his legs, or starting to mount, find the Voice of God within yourself, the deepest darkest voice you can call up from the bottom of your belly, and yell NO! at him. Think barking Mastiff. Go for that voice. Stand up immediately, square off your shoulders, put your elbows up, stand like you mean business, and take a step toward him. He should be cowering, tail down, and backing off from you by now. Then make your dog SIT. This will teach your dog STOP what you're doing and OBEY ME.

Throwing things at him or swatting him would teach him 'when you do that, i become aggressive and dangerous'. That's not a good lesson. Putting him outside would only accomplish one thing - getting him out of your hair and off your female pug's back.

If you do this and he does obey by sitting, keep your ground for a while, breathing, watching him. My dog usually sits there with his ears down and his tail down looking like a kid that's been scolded. If you're satisfied with his behavior after 45 seconds to a minute, you can release him from his sit by a deep noise, i usually grumble OK or let out a rumbling hrmmm noise, and nod, and then walk off to go do something else.

It's happened that my dog immediately returns to his misbehavior, in which case the voice comes back again for a much stronger correction this time (usually 'oh NO you did NOT', haha), a much more obvious and visible physical response, and the display of submission i require before i release him is much more extensive.

And of course i make sure to give him lots of attention and affection when his behavior is appropriate!