First, to answer your question with a question: Who told you he can gain custody under such circumstances? It may be unlikely, but it is possible. You should consult with a lawyer to prepare your case for custody.
In making a custody detemination, the court tries to determine the "best interests of the child." In addition to the factors mentioned in another answer, judges in Maryland consider (1) Capacity of parents to communicate and reach shared decisions affecting child's welfare, (2) Willingness of parents to share custody, (3) Relationship between child and each parent, (4) Potential disruption of child's social and school life, (5) Demands of parental employment, (6) Sincerity of parent's request, (7) Financial status of parents, and (8) Benefit to parents. Finally, a judge will look at where there has been abuse by a parent against the child, the other parent, or any child in the household.
REMEMBER: In any child custody case, the paramount concern is the best interest of the child. Formula solutions in child custody matters are impossible because of the unique character of each case, and the subjective nature of the evaluations and decisions that must be made. The best interest of the child is therefore not considered as one of many factors, but as the objective to which virtually all other factors speak.
Finally, what are you waiting for? If your spouse has committed adultery, you don't need to wait to file for divorce. Please see a lawyer immediately. Good luck.

Below are the factors the court considers when determining what custody arrangement is in a child's best interest:

Primary Care Giver - Who is the person who takes care of the child? Who feeds the child, shops for their clothes, gets them up for school, bathes them, and arranges day care? Who does the child turn to when they get hurt?

• Fitness - What are the psychological and physical capacities of the parties seeking custody? The court may also consider evidence of abuse by a party against the other parent, the party's spouse, or any child residing within the party's household (including another child).

• Character and Reputation

• Agreements - Is there a custody agreement already in place?

• Ability to Maintain Family Relationships - Who will be best able to help the child keep family relationships? Who is going to let the child speak with their ex-mother-in-law, for example? Who will not penalize the child for any bad action on the part of the other parent?

• Child Preference - The decision of the court may be reversed on appeal if the judge will not hear the child's preference. However, the judge may choose to interview the child outside the presence of the parents. A child as young as 5 or 6 years of age may be heard. Though it is rare, the court will hear from a child under 7 years. The child's maturity, and whether the child can tell the truth from fiction will guide the decision whether a child may be heard. A child of at least 10 or 12 years of age is certainly entitled to have their opinions heard and given weight in legal proceedings about custody. Additionally, the court has the power to appoint an attorney for the child in contested cases.

• Material Opportunity - Which parent has the financial resources to give the child more things?

• Age, Health and Gender of Child

• Residences of Parents and Opportunity for Visitation - How close do the parents live to each other? How close do they live to members of the child's extended family? Which parent lives closest to the child's school and social circle?

• Length of Separation- how long has the parent been separated from the child?

• Any Prior Abandonment or Surrender of Custody - Is there a history of one parent walking out and leaving the other parent to cope with the child and the home? Which parent left when you last broke up?

• Religious Views - These will be important in the court's decision only if you can show that religious views affect the physical or emotional well being of the child.

• Disability - A party's disability is only relevant to a custody decision if the disability affects the best interest of the child.