DFM1, have no worries, you did not start a fued. My response has merely revived a long standing fued that most of the respondents to inquiries to this Bulletin Board have encountered with Mr. Tieger. It is a shame that many expert plumbers do not respond to inquiries such as yours simply because they do not want to be the target of insults hurled by Sylvan. Sylvan is a very accomplished person when it comes to plumbing issues, but his interpersonal skills suck when he disagrees with the responses submitted. The insults hurled by him do not always make his opinions right. They just reveal his insecurities, and his inability to stay focused on the problem at hand.
My goal in responding to inquiries is to answer the questions to the best of my ability, and I do not submit responses unless I am absolutely sure that I can be of some assistance. I simply ignore his self-important attitude, and ignore his insults, recognizing that most intelligent people can identify his peculiar personality. At least he does not use profane language, unlike that ignorant pig, Hobart Mendoza.
That said, lets address your dilemma. In your original inquiry you stated the following:
>I have a sump pump in my basement that has the bathroom sink, shower and washing machine draining into it.<
Well, the flows from these plumbing fixtures are considered sewage. These flows are generally referred to as greywater, but they do contain varying amounts of organic matter. The odors that you are smelling are a product of the anaerobic microbial digestion of organic matter in the sewage. So the answer to your question regarding the necessity of venting the sump pump pit is yes. It is necessary if you do not wish to continue smelling these foul odors.
Measure the exact dimensions of the opening of the concrete sump pump pit, and visit a local plumbing supply store to search for a plastic lid for this application. Explain to the professional at the store the problem that you are trying to solve. This expert will search the catalogs for the appropriate lid, gaskets, and grommets required, and may instruct you as to the proper procedure for installation.
Alternatively, contact a Licensed Master Plumber to complete the job if you do not feel that you have the skills to perform the required plumbing, and electrical work.